A Century of U-Pb Geochronology

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F. Corfu A century of U-Pb geochronology: The long quest towards concordance

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A century of U-Pb geochronology: The long quest towards concordance

F. Corfu†

Department of Geosciences, University of Oslo, Postbox 1047, Blindern, N-0316 Oslo, Norway

ABSTRACT

The U-Pb system is a prime geochronom-eter, mainly due to its occurrence as a pair of isotopically distinct but chemically identical decay systems with 235U decaying to 207Pb and 238U to 206Pb, respectively. In addition, U has suitable half-lives and is hosted in some very convenient minerals, such as the widespread and robust mineral zircon. These twin decay systems, running at different speeds, allow an immediate verifi cation of the validity of their ages, which must be concordant to be consid-ered valid, although under favorable circum-stances, discordant data can be extrapolated to the correct age. In detail, the degree of dis-cordance can vary greatly, and discordance can have many different causes. The progress in developments of the U-Pb method is both a history of technical discoveries and advances, as well as a history of a long struggle toward concordance, toward an understanding of the causes of discordance, and toward ways to eliminate it. Despite the enormous progress achieved in this fi eld, the problems of U-Pb discordance have not yet been completely resolved and will be one of the main hurdles to overcome in the future. This paper reviews some of the main stages in the evolution of the method, focusing especially on the style, causes, and implications of U-Pb discordance in modern geochronology.

Only one answer is the right one, and onlythat one is good enough. —T.E. Krogh

INTRODUCTION

The development of U-Pb geochronology is rooted in the early days of discovery of radio-activity and the radioactive decay of U with emission of alpha and beta particles and a grad-ual accumulation of radiogenic Pb. A descrip-tion of the most fundamental discoveries and of the progressive development of mass spectrom-etry is given in some detail in earlier reviews,

such as that by Davis et al. (2003), and will not be repeated here. Research on this subject during the fi rst 50 yr of the twentieth century prepared the ground for the emergence of U-Pb geochronology as a robust dating technique. The fi rst wave of applications in the 1950s de-veloped basic techniques, undertook the fi rst geological case studies, and discovered the real-ity of U-Pb discordance. The second and subse-quent waves searched for ways to accommodate U-Pb discordance and explored various strate-gies to eliminate it. Now, 60 yr later, we see two contrasting trends, two philosophies: one that continues the battle to eliminate discordance, but also a second one that bypasses the prob-lem and solves the Gordian knot by essentially eliminating the concept, declaring U-Pb sys-tems concordant by defi nition (see section “Ad-ministrative Concordancy”). This paper follows these historical developments. A brief descrip-tion of the U-Pb method and specifi c technical aspects is given in an Appendix in the GSA Data Repository.1 More detailed descriptions of various aspects of the analytical technique and error analysis can be found in Parrish and Noble (2003), Bowring and Schmitz (2003), and Con-don and Bowring (2012).

EVOLUTION OF THE TECHNIQUE

The Pioneers—Realization of U-Pb Discordance

Following the discovery of radioactivity, and after several decades dedicated to research into fundamental aspects of radioactive systems, a critical advance was the development of mass spectrometry. The fi rst measurements of Pb isotopic compositions by Nier (1939) were an important analytical step in geochronology, which up to then had been based on bulk chem-ical ratios. In a review of the methods current by ca. 1950, and of the data obtained by then,

Kulp et al. (1954) pointed out that analyses tended to give discrepant ages for 206Pb/238U, 207Pb/235U, and 207Pb/206Pb. They discussed the various factors affecting the analyses, consid-ering especially loss of Rn, and concluded that 207Pb/235U was the most reliable, and 207Pb/206Pb was the least reliable age. Ahrens (1955a) used data for monazite from Rhodesia (Zimbabwe) reported by Holmes (1954) to show that the divergence between 206Pb/238U and 207Pb/235U and 207Pb/206Pb changed in systematic ways, and demonstrated graphically the convergence along curved trajectories toward a common age. Discordant data from Manitoba and Mada-gascar could in part be explained by the same scheme, and he interpreted the pattern as be-ing due principally to Pb loss, also establishing that 207Pb/206Pb ages were those closest to the real age, the opposite of the conclusion of Kulp et al. (1954). In a subsequent paper, Ahrens (1955b) presented a semilogarithmic plot of t (time) 207Pb/235U versus log t 206Pb/238U, show-ing that the Rhodesian data fi tted a straight line, and, using preliminary versions of concordia diagrams, he discussed the causes of discor-dance. The modern version of the concordia diagram, together with a detailed mathematical treatment of the effects of loss and gain of Pb and U, was then provided by Wetherill (1956a, 1956b). His formalism became the basis for the subsequent treatment of discordant data (e.g., Russell and Ahrens, 1957; Wetherill, 1963). A different type of concordia diagram was later introduced by Tera and Wasserburg (1972) (see Appendix in the GSA Data Repository [see footnote 1]).

Isotope Dilution–Thermal Ionization Mass Spectrometry (ID-TIMS)

The application by Tilton et al. (1955) of isotope dilution techniques2 to the determi-nation of U, Pb, and Th in minerals was the

For permission to copy, contact [email protected]© 2013 Geological Society of America

33

GSA Bulletin; January/February 2013; v. 125; no. 1/2; p. 33–47; doi: 10.1130/B30698.1; 6 fi gures; Data Repository item 2013061.

1GSA Data Repository item 2013061, Brief descrip-tion of the U-Pb method and specifi c technical aspects, is available at http://www.geosociety.org/pubs/ft2013.htm or by request to [email protected].

2Abbreviations used in this paper: ID-TIMS—iso-tope dilution–thermal ionization mass spectrometry; SIMS—secondary ionization mass spectrometry; (LA-)ICP-MS—(laser ablation) inductively coupled plasma–mass spectrometry.†E-mail: [email protected]

Invited Review

CELEBRATING ADVANCES IN GEOSCIENCE

1888 2013



Corfu

34 Geological Society of America Bulletin, January/February 2013

fundamental technical innovation that opened the doors to work with zircon. In the following half century, this mineral became the dominant geochronometer. Tilton and coworkers tested the method using a sample from a granitic rock from the Grenville Province. Zircon gave U-Pb and 207Pb/206Pb ages that were consid-ered equal within the limits of uncertainty. The analysis of coarse titanite also yielded the same 207Pb/206Pb age as the zircon. The procedure adopted in that study was extremely complex and time consuming, seen from our present-day perspective, but it represented a great step forward, permitting analysis of gram quanti-ties of minerals, when much larger quantities had previously been necessary. Besides zircon, Tilton and Grünenfelder (1968) also examined the behavior of titanite (sphene) in different settings, observing a much lower susceptibility to partial Pb loss, although it was more easily reset (or newly formed) during high-tempera-ture events.

Modeling Phase

With improvements in technique and in-creased production of data, attention was fo-cused on the general problems of discordance of U-Pb systems in nature. There was a gradually growing consensus that the dominant cause of discordance was loss of Pb, and different at-tempts were made to rationalize the effect and extract reliable ages. Wetherill (1956b) had pro-vided the basic graphic and mathematical tools for dealing with discordant data. Subsequently, several models were elaborated that linked discordance to various diffusion mechanisms, either simple volume diffusion, diffusion linked to gradual metamictization of the host, or diffu-sion linked to episodic activation (Nicolaysen, 1957; Tilton, 1960; Wasserburg, 1963; Weth-erill, 1963). Pidgeon et al. (1966) conducted a series of experiments using portions of a large metamict zircon immersed in hot NaCl solutions and observed considerable leaching of Pb and a reduction of the Pb-U ratio. Grünenfelder et al. (1964) showed that some of the discordance re-fl ected mixing with old Pb components. Subse-quently, a more sophisticated multistage Pb loss model was added to the toolbox (Allègre et al., 1974). Along a different track, Goldich and Mudrey (1972) had discussed a model of Pb loss by dilatancy with loss of water from microcapil-laries in zircon during uplift. This mechanism is touched on also by a concept more recently discussed by Kramers et al. (2009), who pro-posed that radiogenic Pb forms in the tetravalent state, and hence remains immobile until it is ex-posed to fl uids that change its redox state and make it mobile. They further point out that the

last α-decay in the chain to 207Pb involves more energy and causes more damage than the corre-sponding last decay to 206Pb; hence, the former may be lost preferentially, perhaps causing the commonly observed nonzero lower-intercept ages of discordant arrays of Precambrian zircon populations.

While intellectually stimulating, these models and discussions could not provide unique and fully reliable solutions, and the problems of dis-cordant U-Pb data remained a challenge (e.g., Kouvo and Tilton, 1966; Steiger and Wasser-burg, 1966, 1969).

New Strategies to Understand and Work around Discordance

The rebound from these rather discouraging early phases in the development of U-Pb can be credited largely to the work and imagination of Leon Silver, at Caltech in Pasadena. He real-ized that the weak point in the analytical pro-cedure was “the absence of a suitable approach in selecting samples from which data could be successfully systematized and interpreted” (Sil-ver, 1963a, 1963b, p. 281). He understood that zircon populations in rocks can be extremely variable in terms of composition, crystallinity, and other parameters. He experimented in sepa-rating fractions according to size and magnetic variability, and his measurements of the U-Pb ratios of the different fractions revealed extreme variation in the degree of discordance, which correlated with U content and degree of radio-activity. The U-Pb results defi ned collinear ar-rays for which intercepts could be interpreted in terms of primary crystallization and second-ary Pb loss (Silver, 1963a, 1963b; Silver and Deutsch, 1963). The presence of uranothorite inclusions was also shown to have a strong in-fl uence on the degree of discordance. His tests established that the earlier-discussed Rn loss ef-fect had no great infl uence on the discordance of zircon. He also showed that volume diffusion alone could not explain the pattern, and the loss of Pb required additional controls from the in-ternal parts of the zircon.

Silver’s developments set up analytical ap-proaches that were to become standard proce-dures in the following two decades. Given that discordance was next to unavoidable in most samples, the strategy became that of maximiz-ing the spread in order to establish more robust discordia lines and increase the reliability of their intercept ages. A major hurdle, however, remained the immense analytical effort needed to produce individual analyses from zircon fractions of 100–500 mg (Silver and Deutsch, 1963), which severely limited the utility of the method.

Krogh Revolution 1: Simplifying the Method

The change of focus by Tom Krogh, then at the Carnegie Institution of Washington, to U-Pb geochronology from his previous involvement with Rb-Sr dating (Kamo et al., 2011) repre-sents the next major step in the evolution of the method, and the one that really enabled the more widespread application and testing of the principles and analytical approaches defi ned by Leon Silver’s work.

The main components of the new technique were: (1) the change from fl ux-based decom-position of zircon to hydrothermal dissolution in Tefl on bombs, (2) the chemical separation of U and Pb using ion-exchange resin, and (3) the use of an artifi cial 205Pb isotope as a tracer (Krogh, 1973; Krogh and Davis, 1975a). All chemical steps in this procedure were car-ried out in simple laminar fl ow hoods. In ad-dition, Cameron et al. (1969) had introduced the Si-gel loading technique, which greatly facilitated mass spectrometry due to enhanced ionization of Pb, and Mattinson (1972) had devised an effi cient way to distill the reagents used in the analysis. The main advantages of these combined modifi cations were the drastic reduction (by three orders of magnitude) of the background contamination, much simpler ana-lytical protocols, much smaller sample sizes, and better reproducibility.

The new technique was rapidly embraced by the growing U-Pb community, expanding its application to address the evolution of orogens and decipher the timing of granitic magmatism and metamorphism, but also focusing on spe-cifi c mechanisms causing U-Pb discordance. Good examples of the former are the studies by Pidgeon and Aftalion (1978) on the gran-ites and metamorphic basement of Scotland, Baadsgaard et al. (1976) on early Archean Amitsoq gneisses of West Greenland, Gulson and Krogh (1973) on the Bergell granite, and Van Schmus et al. (1987) on Paleoproterozoic Trans-Hudson orogen. Gebauer et al. (1981) showed that it was possible to fi nd zircon and monazite and date rocks of mafi c and ultra-mafi c composition. Cliff (1980) and Schärer (1980), in part combining the age of different minerals, established the evolution of nappe complexes. In a study of discordance, Grauert et al. (1974) found that a major mechanism af-fecting detrital zircon ages was gain of U dur-ing metamorphism. The reduction in sample size permitted by the new technical advances led to the fi rst single-grain zircon studies (e.g., Lancelot et al., 1976; Schärer and Allègre, 1982). Other studies tested techniques to ex-pand the range of discordance in order to obtain




Geological Society of America Bulletin, January/February 2013 35

better-constrained intercept ages (Aleinikoff, 1983; Steiger et al., 1993). Although this fl urry of activities and new applications helped to solve many geological questions, many of the studies involving zircon had to rely on extrapo-lating the age from discordant arrays, which in complex cases had detrimental effects on the precision and accuracy of the ages.

Krogh Revolution 2: Reducing Discordance

Since model solutions to discordance did not work very well, and since extrapolating the ages from discordant arrays could be problematic in all those cases that had undergone a Pb evolu-tion involving more than just two stages, Tom Krogh concluded that the only reliable strategy had to be that of fi nding, isolating, and analyz-ing concordant domains of zircon. The ground-work laid by Silver and coworkers, and also by others, in the 1960s had shown that discordance is generally not a uniform property of zircon, but is especially prominent in specifi c parts of the grains. By means of leaching and etching experiments, Krogh and Davis (1974, 1975b) discovered that some zircon crystals contain easily dissolvable altered domains (Figs. 1E and 1F). Alteration was caused by fl uids at-tacking the rims of zircon or penetrating along fractures and attacking metamict domains. The fractures formed as a result of tensions caused by volume expansion during metamictization. The penetrating fl uid removed radiogenic Pb while it introduced elements such as Ca and Fe. Because altered domains are easily soluble and could be separated from the unaltered zir-con by partial dissolution, several workers ex-perimented with partial dissolution techniques. The results, however, were not encouraging because the technique introduced secondary ef-fects, fractionating U from Pb in the remaining parts of the zircons (e.g., Pidgeon and Hopgood, 1975; Todt and Büsch, 1981; Turek et al., 1982). Mattinson (1994) reviewed these experiments and concluded that the fractionation of U from Pb was likely related to the precipitation of fl uo-rides during dissolution. Mattinson continued to experiment with, and refi ne these techniques and eventually discovered how to master them a decade later (see later herein).

Krogh (1982a) chose instead a different track. To avoid the side effects of the chemical attacks, he opted to remove the outside of the grains, or of fragments of grains, by spinning them around in a steel chamber with a jet of compressed air (air abrasion method). His tests showed that, in many cases, the technique could indeed strip most of the discordant zircon matter. In a par-allel development, he also experimented with a high-gradient magnetic device that separated

A B

C D

E F

Figure 1. Cathodoluminescence (A–E) and backscattered electron (F) images illustrating typical features of zircon affected by secondary modifi cations causing partial resetting and discordance of U-Pb systems. Scale bar indi-cates 100 μm. (A) Zircon crystal with regular growth zoning, but locally frac-tured and externally resorbed and overgrown by new highly luminescent (and low U) rims during a metamorphic overprint (modifi ed from Corfu, 2007). (B) Small core with regular growth zoning, surrounded by more com-plex regular and sector-zoned domain, itself rimmed by high-luminescence outer layer. The texture indicates overgrowth and new reworking of a mag-matic zircon during two separate high-grade metamorphic events, result-ing in partially reset and discordant data (modifi ed from Corfu, 2007). (C) Zircon crystal with regular and sector zoning, and locally thin rims of low luminescence cut by brittle fractures; the thin rims represent a meta-morphic overprint that caused strong Pb loss, whereas the fractures formed during later mylonitization without causing signifi cant Pb loss (Austrheim and Corfu, 2009). (D) large zircon crystal strained by mylonitization, which caused internal fracturing and local distortion of the lattice and fragmenta-tion of the rims in addition to full recrystallization and/or new growth of ex-ternal neoblastic zircon grains. The strain in this case caused severe resetting of the U-Pb system (from Roffeis et al., 2012). (E) Zircon affected by altera-tion expressed by the black, botryoidal trains of alteration advancing along cracks and interfaces and forcing their way into metamict domains (from Medenbach, 1976). Alteration causes chemical changes and produces severe Pb loss. (F) A similar case, with alteration attacking metamict zones in the zircons and penetrating along cracks perpendicular to the zoning (modifi ed from Nasdala et al., 2010).



Corfu


zircon based on faint paramagnetic properties, thereby eliminating the zircons that had under-gone even very small amounts of alteration, gaining Fe and losing Pb (Krogh, 1982b). In general, to achieve the best results, these meth-ods had to be combined with careful inspection and selection of the grains under a binocular micro scope, and in specifi c cases with imaging (cf. Corfu et al., 2003).

The effi ciency of these treatments was fur-ther greatly improved by the growing abil-ity to measure smaller and smaller samples due to progressive refi nements of the Krogh (1973) technique by miniaturizing dissolu-tion bombs and columns, reducing the blank, which by the end of the 1980s in some labora-tories had started to creep below 1 picogram Pb, and taking advantage of improved mass spectrometers.

High-Resolution ID-TIMS Geochronology

The array of technical improvements, and the development of abrasion, with its much im-proved control on discordance, made it possible to address a variety of geological problems at a level of detail that would not have been pos-sible before. As exemplary applications in that period, one can mention the systematic deter-mination of the chronostratigraphy of Archean and Proterozoic greenstone belts, the dating of ophiolite complexes, the resolution of the em-placement sequence of multistage granitoid complexes and polymetamorphic evolution of lower-crustal terrains, the dating of mafi c dike swarms, and constraining the age of ore deposits and of major meteorite impacts (e.g., Krogh et al., 1987; Dunning and Pedersen, 1988; Davidson and van Breemen, 1988; Davis et al., 1989; Barth et al., 1989; Davis and Smith, 1991; Hans-mann and Oberli, 1991; Corfu and Davis, 1992; Parrish, 1995; Kamo et al., 1996a, 1996b). The new analytical approaches contributed at two distinct levels: on the one hand, by provid-ing accurate ages that permitted correlation of geological elements across belts, across conti-nents, and between continents and, on the other hand, by providing the detailed age resolution necessary to address the speed and kinematics of rock-forming and rock-deforming processes. Besides the ubiquitous zircon, a variety of other minerals could be routinely analyzed, among them, especially titanite and monazite, but also the less common baddeleyite, xenotime, alla-nite, rutile, and perovskite, or the common but less suitable apatite. More specifi c dating appli-cations are those dealing with the formation of very young rocks, such as the progressive depo-sition of opal in veins at Yucca Mountain studied by Neymark et al. (2000).

Kober Technique: Zircon Evaporation

An interesting alternative for dating zircon was developed by Berndt Kober in Heidelberg. It involved mounting a zircon directly on a Re-fi lament and then, after inserting it into a mass spectrometer, subjecting it to rounds of progres-sive heating, which released Pb, and depositing it on a facing cold fi lament, from where it could subsequently be ionized and measured (Kober, 1986, 1987). This procedure allowed, in prin-ciple, the extraction and purging of disturbed Pb from discordant domains before reaching the more fi rmly held undisturbed Pb. In the ideal case, the extraction procedure would provide a number of sequential steps, allowing verifi ca-tion of whether the composition remained con-stant, suggesting a closed system, or changed progressively, suggesting that Pb was disturbed because of loss or mixing with older compo-nents. An additional benefi t of the technique was the fact that it only required a thermal ion-ization mass spectrometer without facilities for low-blank chemical dissolution and separation. The technique was used successfully in a num-ber of studies (e.g., Kröner et al., 1991; Jaeckel et al., 1997). Its main drawback, however, is the fact that it provides only Pb-Pb but no U-Pb information, and hence no direct way to verify the degree of discordance. The measurement procedure is also relatively time consuming. These disadvantages have limited the number of applications of this technique. Recently, a modi-fi ed version has been proposed that promises to provide a better control of the isotopic fraction-ation and thus contribute to high-age-resolution geochronology (Davis, 2008).

The Ion Probe: The Early Challenges and Progress

Around 1980, ID-TIMS was joined by sec-ondary ion mass spectrometry (SIMS). Fol-lowing early steps in Cambridge and Chicago, the main contribution for U-Pb dating was the development of the sensitive high-resolution ion microprobe (SHRIMP) at the Australian National University (ANU) in Canberra by Bill Compston and his group (Williams, 1998). The original attempts to use regular ion probes for U-Pb dating (e.g., Hinton and Long, 1979) encountered diffi culties because of problems correcting for molecular interferences on the desired Pb isotope masses. The SHRIMP main solution was to build a large ion microprobe that was capable of achieving high mass resolution without having to compromise the sensitivity. The SHRIMP instrument attracted a large in-terest from the community, mainly because of the high spatial resolution, which permitted the

analysis of discrete domains of polished sec-tions through grains, with the further advantages of a high analytical speed and the preservation of most of the analyzed objects. When com-pared to ID-TIMS, the main disadvantage of the ion probe was a considerable decrease in preci-sion, and hence loss of temporal resolution. This stemmed mainly from the much smaller amount of material available for measurement. Another critical step was the determination of the U-Pb ratios, which in SIMS must be calibrated against the corresponding ratios of an external standard, in contrast to ID-TIMS, where the ratios are obtained by mixing a tracer of known compo-sition with the sample. Because the sputtering of U and Pb is controlled to some degree by the composition and/or structure of the sample (McLaren et al., 1994), discrepancies can arise between sample and standard when they have quite distinct basic properties. The anomalous behavior is evident in some zircons very rich in U, such as the cases reported by Harrison et al. (1987) and Wiedenbeck (1995). This potential problem is now largely recognized in the SIMS community, and modern analytical protocols are generally concerned with proper matching of the reference zircons.

Ion probe dating has been applied to the en-tire range of terrestrial geological problems, but especially to the study of multistage crustal growth and metamorphism, the genesis of gran-ites, and the sources of xenocrystic and detrital zircons (e.g., Williams, 1998, 2001; Williams et al., 1984; Kinny and Dawson, 1992; Nut-man et al., 1996). One of the most celebrated studies was the discovery of ≥4-b.y.-old zircon grains in conglomerates at Mount Narryer and Jack Hills in Western Australia (Froude et al., 1983), which spawned many other studies on these rare and very critical earliest witnesses of Earth evolution (e.g., Compston and Pidgeon, 1986; Wilde et al., 2001). Special applications of the SIMS technique were also those focused on the rare zircons in meteorites and in rocks brought back from the moon by the Apollo mis-sions (Ireland and Wlotzka, 1992; Nemchin et al., 2008, 2009).

LA-ICP-MS Technique: Problems and Solutions

The latest addition to the arsenal of U-Pb dating methods is the laser ablation–induc-tively coupled plasma–mass spectrometry (LA-ICP-MS) technique. The fi rst experiments with U-Pb dating by LA-ICP-MS were done in the early 1990s (Fryer et al., 1993), and from there the technique rapidly improved. In less than a decade, many laboratories worldwide had adopted it, contributing to many improve-





ments and refi nements . The method is attractive mainly because of the high throughput capabil-ity, which makes it ideal for the study of large populations, such as in detrital zircon studies. The precision tends to be comparable to that achieved by SIMS, but the latter has the advan-tage of consuming much less of the target (spots of 25 μm × 3 μm for SIMS vs. 30 μm × 30 μm for LA-ICP-MS, but these volumes can vary considerably depending on the task envisaged and the operator choices). The main initial chal-lenges for LA-ICP-MS were achieving control of the analytical fractionation between U and Pb, and correcting for common Pb, due to the pres-ence of mercury in the argon gas used to produce the plasma that causes an isobaric interference by 204Hg on 204Pb. Different groups have adopted different strategies for coping with these prob-lems (Košler et al., 2002; Horn et al., 2000; Jack-son et al., 2004; Gehrels et al., 2008; Cottle et al., 2009). The LA-ICP method is now widely used to address many different geological problems, although the limited precision makes it most at-tractive for the study of detrital zircon populations (e.g., Košler et al., 2002; Gerdes and Zeh, 2006).

Chemical U + Th–Total Pb Dating

Chemical dating is based on the determina-tion of the ratio between the total amount of radiogenic Pb and U + Th. This method had represented one of the initial steps into U-Pb dating in the early decades of the twentieth cen-tury, but after the advent of effi cient isotopic measurement techniques around 1940–1950, chemical dating had all but fallen into disuse. Thanks to the technical progress of modern electron microprobes, and the consequent in-crease in sensitivity, the method has now under-gone a considerable revival (Suzuki and Adachi, 1991; Montel et al., 1996; Crowley and Ghent, 1999; Goncalves et al., 2005; Jercinovic and Williams, 2005). The two basic requirements of the method are (1) closed-system behavior, i.e., the system has not lost or gained U, Th, or Pb since its formation, and (2) absence of ini-tial Pb. Because it is not based on the measure-ment of isotopes, the chemical method cannot take advantage of the information from the two separate decay chains of 235U to 207Pb and 238U to 206Pb, with chemically identical pairs of parent and daughter isotopes but different decay speeds, which act as independent chro-nometers and enable us to evaluate whether or not the system has remained closed. The main area of application of chemical dating has been monazite, largely thanks to its very high levels of U and especially Th, and hence Pb, which fa-cilitate the measurement. In addition, monazite is much less prone to Pb loss than zircon, and

hence more likely to be concordant or nearly so (Parrish, 1990). The main advantage of chemi-cal dating over other methods is the ability to analyze very small areas of minerals (5 μm or less), helping to connect the obtained age to the structural and petrologic context.

Mattinson: Development of Chemical Abrasion

As mentioned earlier herein, the early at-tempts to remove discordant domains from zircon by means of leaching or partial dissolu-tion were not very successful, mainly because the process introduced some artifi cial fraction-ation of U from Pb, presumably due to the pre-cipitation of fl uorides during dissolution. Jim Mattinson in Santa Barbara understood this (Mattinson, 1994) but continued nonetheless to experiment with partial dissolution until he eventually succeeded, developing a new reli-able technique. The critical element in chemical abrasion is the fact that crystalline zircon with a low dose of radioactivity-induced defects can be thermally annealed (at temperatures of 800–1000 °C), recovering the original zircon lattice, in a period of time over which it is insuffi cient to repair the damage of much more strongly me-tamict domains (Mattinson, 2005, 2011). Sub-sequent exposure to hydrofl uoric acid dissolves the metamict domains much more easily than the annealed crystalline zircon components, thus making it possible to isolate the latter. An-nealing is important, not only because it protects the crystalline parts from easy dissolution, but especially because it immobilizes the Pb and U in the crystalline parts, preventing the small-scale leaching that was an important cause of U-Pb fractionation in the original experiments. A more subtle effect of the original partial disso-lution experiments was the preferential leaching from crystalline zircon of “young” Pb (Corfu, 2000), separating it from the earlier-produced Pb, which may have been frozen-in during earlier geological annealing events (Davis and Krogh, 2000). The effect could also be caused by the fact that the 238U chain has one more alpha emission than the 235U chain, potentially making 206Pb more leachable than 207Pb (but, as mentioned already, the opposite effect has been suggested by Kramers et al. [2009] in consider-ation of the more energetic last decay in the 235U compared to the 238U chain).

In the decade since its development, chemical abrasion has taken a fi rm place in the toolbox of most ID-TIMS laboratories and has un-doubtedly contributed to improve the quality of many geochronological studies (e.g., Schalteg-ger et al., 2008; Schoene et al., 2010a, 2010b), which also benefi t from improved emitters

(Gersten berger and Haase, 1997). As pointed out by Mattinson (2011), the method only works if the zircon grains include some domains that have remained closed systems throughout their history; it cannot restore a lost age. In my ex-perience, it is generally diffi cult to apply the method to U-rich and metamict grains because they dissolve too easily, even in cases where the grains, though rich in U, are free of alteration and yield concordant data when treated with air abrasion (e.g., Svensen et al., 2012).

Pb-Pb Dating

The Pb-Pb method is a special branch of U-Pb geochronology, but it has been used mostly to pursue a different type of problem, namely, employing the Pb composition of U-poor or U-free minerals and rocks to study the gen-esis of rocks rather than to date them. Occa-sionally, however, the two tracks overlap and complement each other.

One of the most prominent applications of Pb isotopic analysis to dating was that of Patterson (1956), who determined the fi rst (essentially) correct age of 4.55 Ga for meteorites and, by extension, the planet Earth. Subsequent work, combined with U-Pb and other isotopic systems, and with progressive improvements of the tech-niques, has greatly refi ned the scale of details of the chronology of the early solar system (e.g., Tilton, 1973; Tatsumoto et al., 1973; Tera and Wasserburg, 1975; Allègre et al., 1995; Amelin et al., 2002, 2009).

Terrestrial applications of Pb-Pb techniques have been less notable from the perspective of precise dating, focusing mainly on genetic ques-tions. An interesting line of experiments ex-ploring geochronological applications involved differential leaching and dissolution of silicates. These experiments yielded important insights into the distribution of U and Th inside specifi c minerals and into their control on Pb evolution and closed-system behavior of potential chro-nometers (e.g., Frei and Kamber, 1995; Frei et al., 1997). The main handicap of the method has been the necessity to rely solely on the Pb composition, without assistance from U-Pb, thus limiting the ability to evaluate the state of concordance of the analyses. Another interesting application of the Pb-Pb method is the dating of ancient limestones (Moorbath et al., 1987; Jahn and Cuvellier, 1994; Bau et al., 1999).

THE ROAD TOWARD CONCORDANCE: A UTOPIAN GOAL?

As reviewed earlier herein, after the discov-ery of radioactivity a century ago, progress in U-Pb geochronology has been enormous, with



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the development of different methods that have been tested and refi ned, some dropped and picked up again and further expanded. At present, U-Pb dating is the method of choice in many studies of magmatic systems, metamor-phism, and provenance. Thousands of studies have examined the behavior of the decay sys-tem, the behavior of U-Pb in different minerals, and the response in different geological settings. The preoccupation with discordance, which laid a cloud of skepticism over the initial enthusi-asm, has been largely dispersed by the subse-quent developments. Today, we have reasonably good control of the behavior of U-Pb systems. Yet, do we really understand discordance; do we really know how to navigate around it?

Concordant U-Pb ages are “qualitatively, … analyses in which the 206Pb/238U, 207Pb/235U, and 207Pb/206Pb ages are equal within analytical error , or, visually, U/Pb and Pb/Pb ages whose corre-sponding isotope ratios have a 95% confi dence error ellipse that (to a greater or lesser degree) overlaps the concordia curve” (Ludwig, 1998, p. 665; Fig. 2A). Although the basic defi nition of concordance is clear-cut, as soon as the ana-lytical error is considered, the term becomes somewhat fuzzy, occasionally progressing into extreme fogginess.

Near-Concordance

In modern high-precision ID-TIMS studies, it is common to observe that high-quality, highly reproducible sets of U-Pb analyses do not plot exactly on the concordia curve but variously to the right of it (Fig. 2B; e.g., Kamo et al., 1996a; Schmitz and Bowring, 2001; Parrish and Noble, 2003; Schoene et al., 2006). An important rea-son for this behavior is the apparently slightly incorrect U decay constant (Jaffey et al., 1971) as discussed by Schoene et al. (2006). Correct-ing such data with a revised decay constant for 235U, as proposed by Mattinson (2010), can often shift the concordia curve to match the data much more closely. However, even the use of the bet-ter matching decay constant does not always succeed in bringing data sets into concordance.

Another factor that can affect the degree of discordance is the U isotopic composition. Early studies had shown that 238U/235U in zircon was homogeneous within 0.5% (Doe and Newell, 1965), and in the following half a century all laboratories used the value of 238U/235U = 137.88 recommended by Steiger and Jäger (1977). However, more detailed modern investigations show that the ratio is likely too high, and mea-surements of various reference materials give values that are up to 0.08% lower (Condon et al., 2010; Hiess et al., 2012). The effect of such a change on the calculated ages is relatively minor

for relatively young minerals (causing an effect of ≤1 m.y.), but it becomes a more important ele ment in meteorite studies, especially because some of the earliest solar system materials show larger deviations of up to 0.4% in the 238U/235U ratio, attributed to the decay of 247Cm, and re-sulting in changes of the calculated 207Pb/206Pb ratios of up to 5 m.y. (Brennecka et al., 2010).

A second set of causes that has been widely discussed, and convincingly proven in some cases, is related to initial disequilibrium in the decay chains from U to Pb. Incorporation of excess 230Th at the time of formation of a min-eral results in excess 206Pb with data that plot re-versely discordant (Figs. 2A-I). This behavior is very evident in young monazites (Schärer, 1984; Parrish, 1990) because of the strong geochemi-cal affi nity for Th in monazite (and allanite). If the Th/U ratio of the magma is known or can be approximated, the effect can be corrected.

The alternative is to use the 207Pb/235U age alone, which is not affected by 230Th excess. For zir-con, the behavior is generally the reverse, be-cause the mineral forms mostly with a defi cit of 230Th, which then translates into a defi cit of 206Pb (but there are exceptions, such as high Th/U zir-con in carbonatites; Amelin and Zaitsev, 2002). The effect from 230Th defi cit is rather minor; for example, in the 28 Ma Fish Canyon tuff zircons investigated by Schmitz and Bowring (2001), the correction causes a shift of ~0.08 m.y.

A related phenomenon is the excess of 231Pa, which produces an excess of 207Pb (Fig. 2A-I). The phenomenon has been demonstrated con-vincingly in two cases (Parrish and Noble, 2003; Anczkiewicz et al., 2001) where the ef-fect is very strong, but it has been discussed as a possible cause in many other cases where the discordance toward too high 207Pb/235U ratios for uniform 206Pb/238U is distinct, but not extreme.

206 P

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8 U

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concordant

age 2age 1

reverselydiscordant

projectedfrom the originof the concordiacurve (= 0 Ma)

projectedfrom an ancientlower intercept age

discordia line -two-stage evolution

discordant

discordant

A-I

A-II

A-III

B

Figure 2. Concordia diagram with data points (ellipses representing analytical uncertainty) illustrating some terms and concepts discussed in the text. (A-I) Concordant versus discor-dant analyses. (A-II) Discordia line defi ned by a suite of discordant, and one concordant analyses, where the intercepts refl ect two distinct events that affected the population. (A-III) Interpretation of nearly concordant data assuming ancient partial resetting (age 1 by pro-jection from the time of the early disturbance) or modern Pb loss (age 2 by projection from the origin). (B) Typical example of high-precision isotope dilution–thermal ionization mass spectrometry (ID-TIMS) data (from Schoene et al., 2006) plotting in a cluster to the right of the concordia curve (calculated with the decay constants of Jaffey et al., 1971).





Among the most interesting observations are those of Kamo et al. (1989, 2003), Schmitz and Bowring (2001), Amelin and Zaitsev (2002), and Dunning and Hodych (1990). Crowley et al. (2007) documented an effect corresponding to ~50 k.y. for the 0.8 Ma Bishop Tuff zircon. A distinct deviation of 207Pb/235U in baddeleyite has also been interpreted, however, in terms of loss of 222Rn rather than 231Pa excess (Heaman and LeCheminant, 2001). In its oxidized form, Pa+5 is predicted to be more enriched with respect to U (Barth et al., 1989). Direct measurements of 231Pa in Holocene zircon crystals suggest, how-ever, that the effect on the ages should be very small (Schmitt, 2007).

In practice, subtle disequilibrium effects can be diffi cult to distinguish from effects caused by other factors, such as small amounts of inheri-tance of xenocrystic material, Pb loss, incorrect common Pb corrections, or simply analytical biases from improper blank corrections, frac-tionation, and instrumental nonlinearities.

Hard-Wired Discordance

More severely discordant data will generally have been produced by either Pb loss or mixing. Solid state diffusion of Pb from zircon is ex-tremely slow and unlikely to be effective at nor-mal crustal temperatures (Mezger and Krogstad, 1997; Cherniak and Watson, 2001); hence, loss of Pb must be accomplished either by extraction of Pb from altered domains by fl uids (Figs. 1E and 1F; Krogh and Davis, 1974, 1975b) or by expulsion and/or intracrystalline redistribution of Pb from the lattice during deformation and recrystallization (Figs. 1A–1D; e.g., Pidgeon et al., 1998; Connelly, 2001; McFarlane et al., 2005; Reddy et al., 2006). The former mecha-nism generally implies the parallel existence of concordant and discordant domains, making it possible to isolate the concordant parts by some of the techniques mentioned previously herein, and obtain concordant ages. By contrast, Pb loss by recrystallization and/or redistribution often means that there may not be any parts of a mineral left with preserved close-system relationships and thus that are capable of pro-viding concordant ages. These cases are typi-cal of high-grade metamorphic terrains, or sites affected by meteorite impact, causing shock metamorphism where zircons or other minerals have been very strongly reworked so that only discordant data can be extracted (Fig. 2A-II; e.g., Corfu et al., 1994; Connelly, 2001; Moser et al., 2011). The same phenomenon can in part also be seen in xenocrystic zircon populations where targeted analyses only provide partially reset discordant ages (e.g., Cornell et al., 2000), or in titanite (Tucker et al., 1987, 1990; Krogh,

1994) and in rutile (Blackburn et al., 2011). The results of such thorough partial resetting by re-crystallization, commonly paired with local new growth, are linear arrays of data points between the time of crystallization and that of secondary reworking and/or local new growth (e.g., Krogh et al., 1993; Moser et al., 2009).

Reverse Discordance

Reversely discordant data, i.e., data with too high 206Pb/238U and/or too low 207Pb/235U ages plotting above the concordia curve in Wetherill diagrams (Fig. 2A-I), can be the results of sev-eral processes that remove U or introduce un-supported radiogenic Pb. The phenomenon is not uncommon in minerals such as monazite or allanite, which are prone to alteration and chem-ical mobility (Poitrasson et al., 1996). In addi-tion, such minerals incorporate high amounts of Th, including 230Th, and hence will normally build up excess 206Pb, resulting in reversely discordant data, as discussed before (Schärer, 1984). In zircon, reverse discordance is more rarely observed, most likely because U is an-chored much more strongly in the crystal lattice than Pb. Reverse discordance can be produced artifi cially by SIMS in U-rich zircon in which the sputtering behavior is different from that of

the reference zircon (e.g., McLaren et al., 1994; Wiedenbeck, 1995). Natural reverse discordance has, however, been observed in some particular cases, such as in an Early Archean gneiss from Antarctica where reversely discordant analyses plot along a paleodiscordia array, showing that the discordance is not an analytical artifact but is due to ancient redistribution of U and Pb (Wil-liams et al., 1984). A similar case has also been described by McFarlane et al. (2005), and an-other is shown in Figure 3. Zircons from a gab-bro from the island of Senja in northern Norway plot both normally as well as reversely discor-dant but defi ne a well-constrained discordia line for which the lower intercept corresponds to the time of the Caledonian orogeny, which affected the region, hence demonstrating that a gain of radiogenic Pb in parts of some zircon grains during metamorphism was the likely cause.

Romer (2003) discussed potential mecha-nisms capable of creating reversely discordant data, focusing in particular on the role of alpha recoil in creating small-scale heterogeneities in zircon or other minerals (see also Mattinson et al., 1996), heterogeneities which eventually can result in reversely discordant data due to paleo-annealing events, or laboratory leach-ing processes, or differential sputtering during SIMS analyses.

0.31

0.33

1.57.4

206 P

b/23

8 U

207Pb/235U

1750

1770

1790

1810

1830

1850

Intercepts at424±20 & 1802.3 ± 0.7 Ma

MSWD = 0.30

Figure 3. Example of reverse discordance most likely caused by U loss or Pb gain during a metamorphic overprint, as suggested by the high degree of collinearity of the data and the lower-intercept age of 424 ± 20 Ma, which corresponds to the Caledonian orogeny affecting this region. The zircon population is from a Paleoprotero-zoic gabbro (Hamn gabbro) on the island of Senja, northern Nor-way (Corfu, 2005, personal observation). MSWD—mean square of weighted deviates.



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SOME MODERN DILEMMAS

Role of Xenocrysts, Antecrysts, and Pb Loss in High-Resolution Geochronology

High-precision U-Pb geochronology has now advanced to a stage where it is possible to study in detail the chronology of magma-tism and emplacement processes of complex multistage plutons, defi ning rates and speed of magma emplacement, the time of residence in crustal magma chambers, and the speed of cool-ing, and calibrating accurately the time scale (e.g., Bowring and Schmitz, 2003; Matzel et al., 2006). Disequilibrium dating by SIMS of zir-con in recent magmatic systems also provides fundamental new insights into the evolution of magma chambers (e.g., Reid et al., 1997; Schmitt, 2006). Some magmatic systems de-velop very fast; for example, in the Bishop Tuff, zircons are shown to have crystallized within 5 k.y. at 767.1 ± 0.9 ka (Crowley et al., 2007). More commonly, however, studies of plutonic or volcanic rocks reveal more complicated pat-terns with individual zircon grains defi ning ages spread over longer periods of up to 1 m.y. or more, and thought to refl ect growth during early as well as late stages of the magma evolution, mixing, and crystallization (e.g., Mundil et al., 2001; Miller et al., 2007; Schaltegger et al., 2008; Lima et al., 2012). The main diffi culty in interpreting such U-Pb data is in identifying correctly the reasons for the specifi c distribution of discrete data patterns. In favorable cases, a sample contains a uniform zircon population, which yields a reproducible cluster of data with just subordinate outliers, and hence the age can be reasonably based on the coherent group. In felsic magmas that evolve over a period of time, it is generally the youngest data points that are considered to represent the most likely age of emplacement, whereas the older ones are taken to represent antecryst or xenocrysts formed be-fore fi nal consolidation (Miller et al., 2007). The determination of the trace-element composition of the dated zircon grains yields additional infor-mation that can help to discriminate the genesis of grains with different ages, evaluating whether they may have been linked by fractionation, or, alternatively, mixing processes (Schoene et al., 2010b). One critical question affecting modern studies done with chemical abrasion ID-TIMS is whether Pb loss effects can be completely eliminated by the chemical abrasion procedure. Some examples show that this assumption is not always valid (Schoene et al., 2010a). A similar uncertainty, but in the opposite direction, is in-troduced by the diffi culty of evaluating whether some slightly too old grains might indicate the presence of antecrysts or traces of xenocrystic

material. Finally, the not always easily control-lable role of disequilibrium in 230Th, 231Pa, and 222Rn, discussed previously, can further compli-cate the interpretation of high-resolution data. Ultimately, the overriding criterion for the valid-ity of an interpretation is the mutual coherence of the data, their consistency with the various properties of the analyzed zircons, the consis-tency with coexisting minerals, and the con-sistency within the geological framework (e.g., Schmitz and Bowring, 2001; Ramezani et al., 2007; Shen et al., 2011).

Modern versus Ancient Pb Loss

A somewhat related dilemma, which be-comes more acute in interpreting Paleozoic and older U-Pb data sets, concerns the interpretation of data that are nearly, but not fully, concordant (Fig. 2A-III). One common solution is to extrap-olate to the concordia curve by projecting the data from a lower-intercept age of 0 Ma, which is equivalent to using the 207Pb/206Pb age of the near-concordant cluster and assuming a modern disturbance (age 2 in Fig. 2A-III). The alterna-tive possibility is that the slight discordance is due to ancient Pb loss: In such cases, the ages can be extrapolated by projecting a line through the data from lower-intercept ages correspond-ing to the time of the supposed event (age 1 in Fig. 2A-III). The second choice is generally made based on the evidence from independent geological factors, paired with the trends of the data and the behavior dictated by comparable data patterns in related units. In some cases, the choice is strongly supported by the combined available evidence, whereas in other cases, it is very much open to debate and often decided on the basis of the interpreter’s bias.

An interesting example of this dilemma was reported by Stern et al. (2009), where air-abraded zircon yielded a main group of 0.9% discordant data with a 207Pb/206Pb age of 3465.4 ± 0.6 Ma, but two chemically abraded zircon grains yielded data points closer to con-cordia and with a slightly higher 207Pb/206Pb age of 3467.1 ± 0.6 Ma, which overlapped the upper-intercept age of a line through all the analy ses of abraded zircons, including a few as much as 3% discordant.

The question as to whether discordant data should be interpreted in terms of modern or an-cient disturbances can also have a large impact on the use of Hf isotopic compositions in zircon. Recent advances in the analysis of Hf isotopic compositions using multicollector ICP-MS techniques have greatly improved the opportu-nities to study crustal growth processes, espe-cially those concerning the earliest evolution of Earth. These studies employ zircon, which can

be dated by U-Pb and contains much Hf (1%) and little Lu, therefore facilitating the determi-nation of initial Hf isotopic compositions. One approach is to dissolve the zircon and separate U, Pb, and Hf before the measurement (e.g., Amelin et al., 1999, 2011). The alternative is to analyze parts of the minerals with LA-ICP-MS or using combinations of ion probe and ICP-MS (e.g., Harrison et al., 2005). One of the most re-cent techniques developed is based on the de-termination, by LA-ICP-MS, of the Hf isotopic composition concurrently with the 207Pb/206Pb ratio in the same targeted zircon domain. The technique has the advantage of linking the Hf composition to the age of the same specifi c parts of multistage zircon and thus makes it possible to evaluate mixing relationships and the compo-sition of end members (Kemp et al., 2009). The use of 207Pb/206Pb ages, however, can be prob-lematic if the data are discordant, and the discor-dance is related to ancient disturbances rather than modern Pb loss. Whereas new growth of zircon involves the buildup of layers with lower 207Pb/206Pb but distinct, generally higher, 176Hf/177Hf, ancient Pb loss from zircon will re-duce the 207Pb/206Pb ratio without affecting the Hf composition (Corfu and Noble, 1992; Kemp et al., 2010).

The Hf isotopic data are commonly discussed in frames of reference defi ned by the chondritic evolution, whereby 176Hf/177Hf ratios are trans-formed into deviations from the chondritic value at their specifi c age. Because of the dependence of εHf on the age, calculations with ages that are too low or too high will result in erroneous εHf values. This relationship can be illustrated using an example from Kemp et al. (2010), who re-ported ages and Hf compositions of Archean detrital zircons from the Jack Hills conglom-erates in Western Australia. The study utilized zircon initially dated in multiple spots with the ion probe, yielding precious information into the U-Pb behavior of each grain. Subsequent analyses were carried out with LA-ICP-MS by measuring Hf and Pb concurrently on the same zircon spots. Two examples are discussed here. Grain jh17–7 was analyzed in three spots with the ion probe, yielding the three U-Pb data points shown in Figure 4A. The three analyses are concordant within error, but their 207Pb/206Pb ages show a distinct dispersion between 3901 Ma and 4029 Ma, indicative of an ancient dis-turbance. The subsequent LA-ICP-MS analy-ses, done in the same spots, yielded a similar spread in 207Pb/206Pb age from 3941 to 4067 Ma. Without further information, it is not possible to construct an accurate projection to the real age. For the sake of discussion, discordia lines are projected through the three analyses from arbi-trary lower-intercept ages of 2000 Ma and 3000





Ma, yielding upper-intercept ages of 3994 Ma and 4055 Ma, respectively, where the probabil-ity of fi t improves for the older lower-intercept age. The merits of this projection are also sup-ported by the fact that the upper-intercept age best matches the oldest reported 207Pb/206Pb. In Kemp et al. (2010), the Hf data were calculated using the concurrent 207Pb/206Pb ages, yielding distinct εHf values of –7.7 to –3 (Fig. 4B). By contrast, calculation of εHf using the extrapolated upper-intercept age of 4055 Ma yields values of –4.6 to –5.5, which overlap each other within error. The second example, grain jh17–26, is more extreme. Here, the ion probe data yielded two discordant data points with slightly diver-gent 207Pb/206Pb ages of 4054 Ma and 4031 Ma (Fig. 4A), whereas the 207Pb/206Pb ages obtained concurrently with the Hf composition by LA-ICP-MS gave 4015 Ma and 3841 Ma. Hence, there is again evidence for nonzero Pb loss. In this case, making an extrapolation to concordia is much more uncertain. Use of a lower-inter-cept age of 3000 Ma leads to an impossibly old age, whereas a lower-intercept age of 2000 Ma yields an upper-intercept age of 4280 Ma. The εHf calculated using the concurrent 207Pb/206Pb ages shows a large variation between –6.2 and –9.9, whereas the extrapolated age of 4280 Ma

produces εHf values that are identical within error (Fig. 4B). The lower-intercept age of 2000 Ma is used here only to demonstrate the general concept. The fact that the resulting εHf values are identical is simply due to the fact that the measured 176Hf/177Hf values are also essen-tially identical. In absolute terms, however, the εHf values calculated with an inferred primary (= concordant age) are dramatically different (giving slightly positive values) from those cal-culated using the discordant 207Pb/206Pb ages.

It is not the purpose of this section to advo-cate specifi c ages and argue in favor or against specifi c εHf values for the ancient Jack Hills zircons, but the examples stress the importance of a proper consideration of the reality, and the causes and the effects of U-Pb discordance on the results and their interpretations. In the cho-sen examples, it is clear that the dispersion of the calculated εHf data is both artifi cially en-larged and biased toward too negative values. These cases are representative of the approach followed in a number of studies focused on early crustal evolution, as well as studies that use Hf isotopes in zircon to deal with the younger his-tory of Earth. Evidently simply ignoring discor-dance distorts the reality, because, as the present examples show, discordance matters.

Subconcordant Data and the Handicap of Low Precision

The “hard-wired” discordance discussed in the previous section can produce linear arrays of data points, which, when the difference be-tween the two intercept ages is small enough, become subparallel to the concordia curve and, dependent on the magnitude of the uncertainty, indistinguishable from it (e.g., Corfu et al., 1994; Corfu, 2007). Resolving whether a set of data defi nes a subconcordant linear array representing a single disturbed, or two mixed, generations, or whether it represents a random distribution of dates, each representing the con-cordant age of a particular grain, is largely a function of the analytical uncertainty.

Such a case is demonstrated by the example in Figure 5 using data reported by Moser et al. (2009). The more precise ID-TIMS data are dis-cordant but plot on a discordia line (Fig. 5A), whereas the less precise SIMS data all overlap the concordia curve within error (Fig. 5B). In this particular case, the two sets of data complement each other very well, and once integrated with the textural information of the zircon grains, and the geological context, they back up a coherent interpretation of two specifi c events. However,

0.7

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0.9

30 40 50 60207Pb/235U

206 P

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8 U

Hf

3300

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4100error ellipses are 68.3% conf

jh17-26.1+2projected from 2000 Ma

4280 ± 77 Ma

projected from 0 Ma4050 ± 9 Ma

MSWD =1.3

MSWD = 3.1

-

jh17-7.1+2+3projected from 3000 Ma

4055 ± 130 MaMSWD = 0.17

projected from 2000 Ma3994 ± 45 Ma

MSWD = 2.0

–12

–8

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0

4

3700 3900 4100 4300 4500age [Ma]

+++

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jh17-7.1+2+3calculated with concurrent

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calculated for 4055 Ma+

approximate2 uncertainty

jh17-26.1+2calculated with concurrent

207Pb/206Pb age

calculated for 4280 Ma

all εHf data calculatedwith concurrent207Pb/206Pb ages

A B

Figure 4. (A) Concordia diagram showing U-Pb spot analyses by secondary ionization mass spectrometry (SIMS) on two zircon grains jh17–7 and jh17–26 from a Jack Hills detrital zircon population (Kemp et al., 2010). (B) Plot of εHf vs. age for the data obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) from the same zircon grains as in A, calculated and plotted using both the concurrent 207Pb/206Pb ages (the procedure followed by Kemp et al., 2010) and the inferred upper-intercept age of the discordant arrays. Use of the 207Pb/206Pb ages of discordant analyses introduces an artifi cial spread in εHf and a signifi cant bias toward more negative values. The fi eld delimited by the stippled line encompasses all the εHf values calculated using the concurrent 207Pb/206Pb ages in Kemp et al. (2010). Abbreviations: DM—depleted mantle, CHUR—chondritic uniform reservoir; MC—mafi c crust; UCC—upper continental crust; MSWD—mean square of weighted deviates.



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taken alone without the supplementary informa-tion, the SIMS data could also be explained in terms of multiple growth periods, or multiple sources if this were a detrital zircon population. In fact, the pattern mimics that seen in many modern studies of complex high-grade terrains, or of detrital zircon populations done by SIMS or LA-ICP-MS, where the data are considered “concordant” as long as they are not more than several percent discordant (often a 5% or 10% cutoff limit is used). For Precambrian zircons, the 207Pb/206Pb ages will generally be used and interpreted by means of probability density dia-grams. Such an artifi cial example, constructed using the data from Moser et al. (2009), is shown in Figure 5C. The statistical treatment extracts the two main age peaks corresponding to the initial crystallization at 2670 Ma and secondary overprint in response to the Vredefort impact at 2023 Ma. However, there are also several other peaks of variable magnitude, which in this case are known to be geologically meaningless. How-ever, in a modern detrital zircon study, such a pattern would normally be interpreted as indicat-ing real ages, and the possibility that individual subconcordant data may be recording partial resetting or mixing is rarely, if ever, considered. This may not be a very important factor in sedi-mentary rocks produced from relatively simple sources, such as an island arc, but it is likely to have serious repercussions on the apparent age distribution produced in zircon derived from more complex high-grade terranes.

Administrative Concordancy

Some contamination by common Pb (and U) is essentially unavoidable in all dating tech-niques. Some common geochronometers such as titanite and apatite, and to some degree also monazite, incorporate some initial Pb. Zircon is generally virtually free of initial common Pb, except crystals with rare compositions, or those that are altered or contain inclusions of Pb-bear-ing minerals. The ID-TIMS method introduces Pb and U contamination during the dissolution, the chemical separation, and the loading on a fi l-ament in preparation for the mass spectrometry. In SIMS and LA-ICP-MS measurements, con-taminant Pb (and U) can be introduced mainly during the mounting and polishing process. Practitioners of all three methods are concerned with keeping the common Pb levels as low as possible, and have devised specifi c techniques to achieve this goal. Nevertheless, it is likely that some contamination will affect every analysis.

In good ID-TIMS laboratories, the Pb blank reaches levels as low as 0.2 pg, i.e., a few femto-grams 204Pb, a level still easily detected in a thermal ionization mass spectrometer. Hence,

ytilibaborpevitaleR

207Pb/206Pb - ages

207Pb/206Pb - age [Ma]2000 2200 2400 2600

C SIMS

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2023 Ma

A ID-TIMS data

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Vredefortcrater

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B SIMS data

Figure 5. Zircon U-Pb data from a mafi c xenolith from the Lace kimber-lite in South Africa. The zircon formed magmatically at 2670 Ma and was strongly disturbed by the processes caused by deep crustal deformation and metamorphism after the Vredefort meteorite impact (Moser et al., 2009). (A) Isotope dilution–thermal ionization mass spectrometry (ID-TIMS) data plot along a discordia line. (B) Secondary ionization mass spectrometry (SIMS) data showing the same trend as the ID-TIMS data but at lower reso-lu tion. Spot analyses show that zircon lost Pb due to crystal-plastic defor-mation, although full resetting was achieved only in a few domains (Moser et al., 2009). (C) Relative probability plot of the SIMS 207Pb/206Pb ages ex-tracting a number of peaks and subpeaks. The initial and the fi nal peaks correspond to the two real events, but the intermediate ones are spurious and geologically meaningless. The example shows that an awareness of dis-cordance and precision are important for accurate interpretations.





ID-TIMS analyses always include the measure-ment of 204Pb, which is then used to correct for the nonradiogenic Pb. Questions affecting the precision are usually related to the proportion of blank versus initial Pb, and to how well one knows, or can estimate, their compositions and uncertainties. These factors propagate into the total uncertainty, and for very small samples they become its dominant element.

In SIMS analyses, measuring the 204Pb peak can be diffi cult because of the very small sig-nal produced during the analysis and because of overlaps with potential isobaric interferences. Thus, besides measuring 204Pb directly and cor-recting as in ID-TIMS, the so-called 207Pb and 208Pb corrections are also used. The former as-sumes concordance and calculates the amount of common Pb needed to shift the measured 207Pb/206Pb to the concordant equivalent. The latter assumes instead that Th, U, and Pb were not disturbed in the mineral and calculates the amount of common Pb from the difference be-tween the measured and the expected 208Pb/206Pb. The 208Pb correction, however, has been shown to introduce signifi cant uncertainties in samples with normal to high Th/U (Williams, 1998). The disadvantage of the 207Pb method is that, for samples that are actually discordant, the dictated administrative concordance will result in an overcorrection and a biased age, and also implies a loss of information on the mutual iso-topic relationships between grains, information which can be important for the interpretation.

In LA-ICP-MS analyses, the main problem is caused by the interference on 204Pb by 204Hg introduced with the Ar gas. It is, in principle, possible to minimize the amount of Hg present and correct for it by measuring the abundance of 202Hg (Gerdes and Zeh, 2009), but most users consider that this correction introduces too much uncertainty, and choose instead to either make no correction at all, based on the assump-tion that zircon has no common Pb and that the blank is negligible, or they correct the com-mon Pb with a model calculation that assumes a coherent behavior of Th/Pb and U/Pb and estimates the time of the isotopic disturbance (Andersen, 2002). Both approaches can have problematic implications for the accuracy of the data and the geological deductions.

Discriminating Origins of Zircon (and Other Minerals’) Based on Th/U

Factors that are important for a proper inter-pretation of U-Pb data include the geological and mineralogical context of the studied miner-als, the internal textures of the minerals, the age relationships between different components of a mineral and between different coexisting miner-

als, the type and spatial arrangement of inclu-sions, and the mineral’s chemical composition. An increasing number of studies have been de-voted to the systematic assessment of the role of trace elements in zircon (Heaman et al., 1990; Belousova et al., 2002; Hoskin and Schalteg-ger, 2003), especially of the rare earth elements (REEs) (Rubatto, 2002; Whitehouse and Platt, 2003) and Ti (Watson and Harrison, 2005; Fu et al., 2008), the latter for geothermometry. An important branch also deals with stable isotopes in zircon (e.g., Valley et al., 2005; Whitehouse and Nemchin, 2008).

One of the most commonly discussed geo-chemical discriminants of U-bearing minerals is the Th/U ratio, because it can be measured directly during isotopic analysis, or it can be estimated from 208Pb/206Pb. The ratio of Th/U is popular mainly because of the widespread per-ception that it can be used to distinguish mag-matic from metamorphic zircon (or titanite) (Bingen et al., 2001; Rubatto, 2002; Kramers and Mouri, 2011). It is indeed correct that mag-matic zircon and titanite quite commonly yield Th/U of 1.0–0.2, whereas metamorphic zircon in amphibolite- and eclogite-facies rocks tends to have ratios of 0.2–0.0. There are, however, many exceptions. For example, very low Th/U ratios are very common in magmatic zircons grown in highly evolved granites, aplites, pegmatites, and leucosomes in migmatites. One factor affecting the ratio is the scavenging of Th by other miner-

als, such as monazite or epidote (Bingen et al., 2001), but another important reason is the fact that Th/U tends to decrease progressively during the evolution of granitic systems (Fig. 6), and hence zircon in highly evolved rocks will simply inherit the low Th/U from the magma. Con-versely, high Th/U ratios can be very prominent in metamorphic zircon and titanite of high-grade rocks (e.g., Moser and Heaman, 1997; Corfu et al., 1994). Hence, it is important to stress the fact that Th/U alone has a very limited diagnos-tic value (Whitehouse and Kamber, 2005; Harley et al., 2007) and should only be considered in combination with other lines of evidence.

FINAL REMARKS

Progress in the development of U-Pb dat-ing techniques has been driven forward by the spirit of technical innovation, but at the same time, it has been moderated by some inherent limitations. Many imaginative approaches had to be developed to make the system work. The recognition of the causes of U-Pb discordance and establishment of techniques to deal with this problem have contributed to the emergence of U-Pb as a prime geochronological tool. Discor-dance, however, remains a general problem, the minimization of which still requires work and creative solutions, unless, out of convenience, the historical consciousness of discordance simply disappears from the collective awareness.

10

100

10010

[ppm]

[ppm]

[ppm]

[ppm]

Th

U

U

Th

1:1

2:14:1Th/U

Giuv

Grimsel

Mont Blanc

VallorcineBergell

CentralAaregranite

aplites

main body

whole-rockvalues

aplite stocks

marginal facies

1

10

1010.1

1:1

3:16:1Th/U

pegmatite

granite

gneiss

schist

++

+

+

Figure 6. Whole-rock Th-U ratios in granitic systems from the central Alps show a very systematic behavior, with both a general increase in U and Th levels during evolution of the systems and a gradual decline in their Th/U ratio (from Rybach, 1973; Rybach and Labhart, 1973). The ratio is especially low in pegmatites and aplites.



Corfu


ACKNOWLEDGMENTS

Encouragements by Editor Brendan Murphy and constructive reviews by Don W. Davis and an anony-mous reviewer are gratefully acknowledged.

REFERENCES CITED

Ahrens, L.H., 1955a, The convergent lead ages of the oldest monazites and uraninites (Rhodesia, Manitoba, Madagas-car, and Transvaal): Geochimica et Cosmochimica Acta, v. 7, p. 294–300, doi:10.1016/0016-7037(55)90039-9.

Ahrens, L.H., 1955b, Implications of the Rhodesia age pat-tern: Geochimica et Cosmochimica Acta, v. 8, p. 1–5, doi:10.1016/0016-7037(55)90013-2.

Aleinikoff, J.N., 1983, U-Th-Pb systematics of zircon inclu-sions in rock forming minerals: A study of armoring against isotopic loss using the Sherman Granite of Colo-rado-Wyoming, USA: Contributions to Mineralogy and Petrology, v. 83, p. 259–269, doi:10.1007/BF00371194.

Allègre, C.J., Albarède, F., Grünenfelder, M., and Köppel, V., 1974, 238U/206Pb-235U/207Pb-232Th/208Pb zircon geochro-nology in alpine and non-alpine environment: Contri-butions to Mineralogy and Petrology, v. 43, p. 163–194, doi:10.1007/BF01134835.

Allègre, C.J., Manhès, G., and Göpel, C., 1995, The age of the Earth: Geochimica et Cosmochimica Acta, v. 59, p. 1445–1456, doi:10.1016/0016-7037(95)00054-4.

Amelin, Y., and Zaitsev, A.N., 2002, Precise geochronol-ogy of phoscorites and carbonatites: The critical role of U-series disequilibrium in age interpretations: Geo-chimica et Cosmochimica Acta, v. 66, p. 2399–2419, doi:10.1016/S0016-7037(02)00831-1.

Amelin, Y., Lee, D.-C., Halliday, A.N., and Pidgeon, R.T., 1999, Nature of the Earth’s earliest crust from haf-nium isotopes in single detrital zircons: Nature, v. 399, p. 252–255, doi:10.1038/20426.

Amelin, Y., Krot, A.N., Hutcheon, I.D., and Ulyanov, A.A., 2002, Lead isotopic ages of chondrules and calcium-aluminum–rich inclusions: Science, v. 297, p. 1678–1683, doi:10.1126/science.1073950.

Amelin, Y., Connelly, J., Zartman, R.E., Chen, J.H., Göpel, C., and Neymark, L.A., 2009, Modern U-Pb chronometry of meteorites: Advancing to higher time resolution reveals new problems: Geochimica et Cosmochimica Acta, v. 73, p. 5212–5223, doi:10.1016/j.gca.2009.01.040.

Amelin, Y., Kamo, S., and Lee, D.-C., 2011, Evolution of early crust in chondritic or non-chondritic Earth inferred from U-Pb and Lu-Hf data for chemically abraded zircon from the Itsaq Gneiss Complex, West Greenland: Canadian Journal of Earth Sciences, v. 48, p. 141–160, doi:10.1139/E10-091.

Anczkiewicz, R., Oberli, F., Burg, J.P., Villa, I.M., Gunther, D., and Meier, M., 2001, Timing of normal faulting along the Indus suture in Pakistan Himalaya and a case of major 231Pa/235U initial disequilibrium in zircon: Earth and Planetary Science Letters, v. 191, p. 101–114, doi:10.1016/S0012-821X(01)00406-X.

Andersen, T., 2002, Correction of common lead in U-Pb analy ses that do not report 204Pb: Chemical Geology, v. 192, p. 59–79, doi:10.1016/S0009-2541(02)00195-X.

Austrheim, H., and Corfu, F., 2009, Formation of planar defor mation features (PDFs) in zircon during coseismic faulting and an evaluation of potential effects on U-Pb systematics: Chemical Geology, v. 261, p. 25–31, doi:10.1016/j.chemgeo.2008.09.012.

Baadsgaard, H., Lambert, R.St.J., and Krupicka, J., 1976, Mineral isotopic age relationships in the polymeta-morphic Amîtsoq gneisses, Godthaab District, West Greenland: Geochimica et Cosmochimica Acta, v. 40, p. 513–527, doi:10.1016/0016-7037(76)90219-2.

Barth, S., Oberli, F., and Meier, M., 1989, U-Th-Pb sys-tematics of morphologically characterized zircon and allanite: A high resolution isotopic study of the Al-pine Rensen pluton (northern Italy): Earth and Plan-etary Science Letters, v. 95, p. 235–254, doi:10.1016/0012-821X(89)90100-3.

Bau, M., Romer, R.L., Luders, V., and Beukes, N.J., 1999, Pb, O and C isotopes in silicifi ed Mooidraai dolomite (Transvaal Supergroup, South Africa): Implications for the composition of Paleoproterozoic seawater and “dat-

ing” the increase of oxygen in the Precambrian atmo-sphere: Earth and Planetary Science Letters, v. 174, p. 43–57, doi:10.1016/S0012-821X(99)00261-7.

Belousova, E.A., Walters, S., Griffi n, W.L., O’Reilly, S.Y., and Fisher, N.I., 2002, Zircon trace-element composi-tions as indicators of source rock type: Contributions to Mineralogy and Petrology, v. 143, p. 602–622, doi:10.1007/s00410-002-0364-7.

Bingen, B., Austrheim, H., and Whitehouse, M., 2001, Ilmen-ite as a source for zirconium during high-grade meta-morphism? Textural evidence from the Caledonides of western Norway and implications for zircon geo-chronology: Journal of Petrology, v. 42, p. 355–375, doi:10.1093/petrology/42.2.355.

Blackburn, T., Bowring, S., Schoene, B., Mahan, K., and Dudas, F., 2011, U-Pb thermochronology: Creating a temporal record of lithosphere thermal evolution: Con-tributions to Mineralogy and Petrology, v. 162, p. 479–500, doi:10.1007/s00410-011-0607-6.

Bowring, S.A., and Schmitz, M.D., 2003, High precision zircon geochronology and the stratigraphic record, in Hanchar, J.M., and Hoskins, P.W.O., eds., Zircon: Reviews in Min-eralogy and Geochemistry, v. 53, p. 305–326.

Brennecka, G., Weyer, S., Wadhwa, M., Janney, J., Zipfel, J., and Anbar, A., 2010, 238U/235U variations in meteorites: Extant 247Cm and implications for Pb-Pb dating: Science, v. 327, p. 449–451, doi:10.1126/science.1180871.

Cameron, A.E., Smith, D.H., and Walker, R.L., 1969, Mass spectrometry of nanogram-size samples of lead: Analytical Chemistry, v. 41, p. 525–526, doi:10.1021/ac60272a020.

Cherniak, D.J., and Watson, E.B., 2001, Pb diffusion in zir-con: Chemical Geology, v. 172, p. 5–24, doi:10.1016/S0009-2541(00)00233-3.

Cliff, R.A., 1980, Uranium-lead isotopic evidence from zircons for lower Palaeozoic tectonic activity in the Austroalpine nappe, the eastern Alps: Contributions to Mineralogy and Petrology, v. 71, p. 283–288, doi:10.1007/BF00371670.

Compston, W., and Pidgeon, R.T., 1986, Jack Hills, evidence of more very old detrital zircons in Western Australia: Nature, v. 321, p. 766–769, doi:10.1038/321766a0.

Condon, D.J., and Bowring, S.A., 2012, A user’s guide to Neoproterozoic geochronology, in Arnaud, E., Halver-son, G.P., and Shields-Zhou, G., eds., The Geological Record of Glaciations: Geological Society of London Memoir 36, p. 135–149.

Condon, D.J., McLean, N., Noble, S.R., and Bowring, S.A., 2010, Isotopic composition (238U/235U) of some commonly used uranium reference materials: Geo-chimica et Cosmochimica Acta, v. 74, p. 7127–7143, doi:10.1016/j.gca.2010.09.019.

Connelly, J.N., 2001, Degree of preservation of igneous zona-tion in zircon as a signpost for concordancy in U-Pb geochronology: Chemical Geology, v. 172, v. 25–39.

Corfu, F., 2000, Extraction of Pb with artifi cially too-old ages during stepwise dissolution experiments on Archean zircon: Lithos, v. 53, p. 279–291, doi:10.1016/S0024-4937(00)00030-X.

Corfu, F., 2007, Multistage metamorphic evolution and na-ture of the amphibolites-granulite facies transition in Lofoten-Vesterålen, Norway, revealed by U-Pb in ac-cessory minerals: Chemical Geology, v. 241, v. 108–128, doi:10.1016/j.chemgeo.2007.01.028

Corfu, F., and Davis, D.W., 1992, A U-Pb geochronologi-cal framework for the western Superior Province, On-tario, in Thurston, P.C., et al., eds., Geology of Ontario: Ontario Geological Survey Special Volume 4, Part 2, p. 1335–1346.

Corfu, F., and Noble, S.R., 1992, Genesis of the southern Abitibi greenstone belt, Superior Province, Canada: Evidence from zircon Hf isotope analyses using a single filament technique: Geochimica et Cosmo-chimica Acta, v. 56, p. 2081–2097, doi:10.1016/0016-7037(92)90331-C.

Corfu, F., Heaman, L.M., and Rogers, G., 1994, Polymetamor-phic evolution of the Lewisian complex, NW Scotland, as recorded by U-Pb isotopic compositions of zircon, titanite and rutile: Contributions to Mineralogy and Petrol ogy, v. 117, p. 215–228, doi:10.1007/BF00310864.

Corfu, F., Hanchar, J.M., Hoskin, P.W.O., and Kinny, P., 2003, Atlas of zircon textures, in Hanchar, J.M., and

Hoskins, P.W.O., eds., Zircon: Reviews in Mineralogy and Geochemistry, v. 53, p. 468–500.

Cornell, D., Årebäck, H., and Scherstén, A., 2000, Ion micro-probe discovery of Archaean and Early Proterozoic zircon xenocrysts in southwest Sweden: GFF, v. 122, p. 377–383, doi:10.1080/11035890001224377.

Cottle, J.M., Horstwood, M.S.A., and Parrish, R.R., 2009, A new approach to single shot laser ablation analysis and its application to in situ Pb/U geochronology: Journal of Analytical Atomic Spectrometry, v. 24, p. 1355–1363, doi:10.1039/b821899d.

Crowley, J.L., and Ghent, E.D., 1999, An electron microprobe study of the U-Th-Pb systematics of metamorphosed monazite; the role of Pb diffusion versus overgrowth and recrystallization: Chemical Geology, v. 157, p. 285–302, doi:10.1016/S0009-2541(99)00009-1.

Crowley, J.L., Schoene, B., and Bowring, S.A., 2007, U-Pb dating of zircon in the Bishop Tuff at the millennial scale: Geology, v. 35, p. 1123–1126, doi:10.1130/G24017A.1.

Davidson, A., and van Breemen, O., 1988, Baddeleyite-zircon relationships in coronitic metagabbro, Grenville Prov-ince, Ontario: Implications for geochronology: Contribu-tions to Mineralogy and Petrology, v. 100, p. 291–299, doi:10.1007/BF00379740.

Davis, D.W., 2008, Sub-million-year age resolution of Pre-cambrian igneous events by thermal extraction–thermal ionization mass spectrometer Pb dating of zircon: Appli-cation to crystallization of the Sudbury impact melt sheet: Geology, v. 36, p. 383–386, doi:10.1130/G24502A.1.

Davis, D.W., and Krogh, T.E., 2000, Preferential dissolu-tion of 234U and radiogenic Pb from α-recoil-damaged lattice sites in zircon: Implications for thermal histo-ries and Pb isotopic fractionation in the near surface environment: Chemical Geology v. 172, p. 41–58, doi:10.1016/S0 009–2541(00)00235–7

Davis, D.W., and Smith, P.M., 1991, Archean gold mineraliza-tion in the Wabigoon Subprovince, a product of crustal ac-cretion: Evidence from U-Pb geochronology in the Lake of the Woods area, Superior Province, Canada: The Jour-nal of Geology, v. 99, p. 337–353, doi:10.1086/629499.

Davis, D.W., Poulsen, K.H., and Kamo, S.L., 1989, New insights into Archean crustal development from geo-chronology in the Rainy Lake area, Superior Province, Canada: The Journal of Geology, v. 97, p. 379–398, doi:10.1086/629318.

Davis, D.W., Williams, I.S., and Krogh, T.E., 2003, Histori-cal development of zircon geochronology, in Hanchar, J.M., and Hoskins, P.W.O., eds., Zircon: Reviews in Mineralogy and Geochemistry, v. 53, p. 145–181.

Doe, B.R., and Newell, M.F., 1965, Isotopic composition of uranium in zircon: The American Mineralogist, v. 50, p. 613–618.

Dunning, G.R., and Hodych, J.P., 1990, U/Pb zircon and baddeleyite ages for the Palisades and Gettysburg sills of the northeastern United States: Implications for the age of the Triassic/Jurassic boundary: Geology, v. 18, p. 795–798, doi:10.1130/0091-7613(1990)018<0795:UPZABA>2.3.CO;2.

Dunning, G.R., and Pedersen, R.B., 1988, U/Pb ages of ophiolites and arc-related plutons of the Norwegian Caledonides: Implications for the development of Iapetus : Contributions to Mineralogy and Petrology, v. 98, p. 13–23, doi:10.1007/BF00371904.

Frei, R., and Kamber, B.S., 1995, Single mineral Pb-Pb dat-ing: Earth and Planetary Science Letters, v. 129, p. 261–268, doi:10.1016/0012-821X(94)00248-W.

Frei, R., Villa, I.M., Nägler, T.F., Kramers, J.D., Przybyl-owicz, W.J., Prozesky, V.M., Hofmann, B.A., and Kam-ber, B.S., 1997, Single mineral dating by the Pb-Pb step-leaching method; assessing the mechanisms: Geochimica et Cosmochimica Acta, v. 61, p. 393–414, doi:10.1016/S0016-7037(96)00343-2.

Froude, D.O., Ireland, T.R., Kinny, P.D., Williams, I.S., Compston, W., Williams, I.R., and Myers, J.S., 1983, Ion microprobe identifi cation of 4,100–4,200 Myr-old terrestrial zircons: Nature, v. 304, p. 616–618, doi:10.1038/304616a0.

Fryer, B.J., Jackson, S.E., and Longerich, H.P., 1993, The application of laser ablation microprobe–inductively coupled plasma–mass spectrometry (LAM-ICP-MS) to in-situ (U)-Pb geochronology: Chemical Geology, v. 109, p. 1–8, doi:10.1016/0009-2541(93)90058-Q.





Fu, B., Page, F.Z., Cavosie, A.J., Fournelle, J., Kita, N.T., Lackey, J.S., Wilde, S.A., and Valley, J.W., 2008, Ti-in-zircon thermometry: Applications and limita-tions: Contributions to Mineralogy and Petrology, v. 156, p. 197–215, doi:10.1007/s00410-008-0281-5.

Gebauer, D., Bernard-Griffi ths, J., and Grünenfelder, M., 1981, U-Pb zircon and monazite dating of a mafi c–ultramafi c complex and its country rocks—Example: Sauviat-sur-Vige, French Central Massif: Contribu-tions to Mineralogy and Petrology, v. 76, p. 292–300, doi:10.1007/BF00375456.

Gehrels, G.E., Valencia, V.A., and Ruiz, J., 2008, Enhanced precision, accuracy, effi ciency, and spatial resolution of U-Pb ages by laser ablation–multicollector–inductively coupled plasma–mass spectrometry: Geochemistry, Geophysics, Geosystems, v. 9, Q03017, doi:10.1029/2007GC001805.

Gerdes, A., and Zeh, A., 2006, Combined U-Pb and Hf iso-tope LA-(MC)-ICP-MS analyses of detrital zircons: Comparison with SHRIMP and new constraints for the provenance and age of an Armorican metasediment in central Germany: Earth and Planetary Science Letters, v. 249, p. 47–61, doi:10.1016/j.epsl.2006.06.039.

Gerdes, A., and Zeh, A., 2009, Zircon formation versus zir-con alteration—New insights from combined U-Pb and Lu-Hf in-situ LA-ICP-MS analyses, and consequences for the interpretation of Archean zircon from the Central Zone of the Limpopo belt: Chemical Geology, v. 261, p. 230–243, doi:10.1016/j.chemgeo.2008.03.005.

Gerstenberger, H., and Haase, G., 1997, A highly effec-tive emitter substance for mass spectrometric Pb iso-tope ratio determinations: Chemical Geology, v. 136, p. 309–312, doi:10.1016/S0009-2541(96)00033-2.

Goldich, S.S., and Mudrey, M.G., Jr., 1972, Dilatancy model for discordant U-Pb zircon ages, in Tugarinov, A.I., eds., Contributions to Recent Geochemistry and Ana-lytical Chemistry: Moscow, Nauka Publishing Offi ce, p. 415–418.

Goncalves, P., Williams, M.L., and Jercinovic, M.J., 2005, Electron-microprobe age mapping of monazite: The American Mineralogist, v. 90, p. 578–585, doi:10.2138/am.2005.1399.

Grauert, B., Seitz, M.G., and Soptrajanova, G., 1974, Uranium and lead gain of detrital zircon studies by isotopic analyses and fission-track mapping: Earth and Planetary Science Letters, v. 21, p. 389–399, doi:10.1016/0012-821X(74)90178-2.

Grünenfelder, M., Hofmänner, F., and Groegler, N., 1964, Heterogenität akzessorischer Zirkone und die petrogra phische Deutung ihrer Uran/Blei-Zerfallsalter II. Präkambrische Zirconbildung im Gotthardmassiv: Schweizerische Mineralogische und Petrographische Mitteilungen, v. 44, p. 543–558.

Gulson, B.L., and Krogh, T.E., 1973, Old lead components in the young Bergell Massif, south-east Swiss alps: Contributions to Mineralogy and Petrology, v. 40, p. 239–252, doi:10.1007/BF00373788.

Hansmann, W., and Oberli, F., 1991, Zircon inheritance in an igneous rock suite from the southern Adamello batholith (Italian Alps): Contributions to Mineralogy and Petrol-ogy, v. 107, p. 501–518, doi:10.1007/BF00310684.

Harley, S.L., Kelly, N.M., and Möller, A., 2007, Zircon and the isotopic record of high- and ultrahigh-temperature metamorphism: Elements, v. 3, p. 25–30, doi:10.2113/gselements.3.1.25.

Harrison, T.M., Aleinikoff, J.N., and Compston, W., 1987, Observations and controls on the occurrence of inher-ited zircon in Concord-type granitoids, New Hampshire: Geochimica et Cosmochimica Acta, v. 51, p. 2549–2558, doi:10.1016/0016-7037(87)90305-X.

Harrison, T.M., Blichert-Toft, J., Müller, W., Albarède, F., Holden, P., and Mojzsis, S.J., 2005, Heterogeneous Hadean hafnium: Evidence for continental crust at 4.4 to 4.5 Ga: Science, v. 310, p. 1947–1950, doi:10.1126/science.1117926.

Heaman, L.M., and LeCheminant, A.N., 2001, Anomalous U-Pb systematics in mantle-derived baddeleyite xeno-crysts from Île Bizard: Evidence for high temperature radon diffusion?: Chemical Geology, v. 172, p. 77–93, doi:10.1016/S0009-2541(00)00237-0.

Heaman, L.M., Bowins, R., and Crocket, J., 1990, The chemical composition of igneous zircon suites: Impli-

cations for geochemical tracer studies: Geochimica et Cosmochimica Acta, v. 54, p. 1597–1607, doi:10.1016/0016-7037(90)90394-Z.

Hiess, J., Condon, D.J., McLean, N., and Noble, S.R., 2012, 238U/235U systematics in terrestrial U-bearing minerals: Science, v. 335, p. 1610–1614, doi:10.1126/science.1215507.

Hinton, R.W., and Long, J.V.P., 1979, High-resolution ion microprobe measurement of lead isotopes: Variations within single zircons from Lac Seul, northwestern Ontario: Earth and Planetary Science Letters, v. 45, p. 309–325, doi:10.1016/0012-821X(79)90132-8.

Holmes, A.H., 1954, The oldest dated minerals of the Rhodesian shield: Nature, v. 173, p. 612–614, doi:10.1038/173612a0.

Horn, I., Rudnick, R.L., and McDonough, W.F., 2000, Precise elemental and isotope ratio determination by simultaneous solution nebulization and laser ablation-ICP-MS: Application to U-Pb geochronology: Chemi-cal Geology, v. 164, p. 281–301, doi:10.1016/S0009-2541(99)00168-0.

Hoskin, P.W.O., and Schaltegger, U., 2003, The composition of zircon and igneous and metamorphic petrogenesis, in Hanchar, J.M., and Hoskins, P.W.O., eds., Zircon: Re-views in Mineralogy and Geochemistry, v. 53, p. 27–62.

Ireland, T.R., and Wlotzka, F., 1992, The oldest zircons in the solar system: Earth and Planetary Science Letters, v. 109, p. 1–10, doi:10.1016/0012-821X(92)90069-8.

Jackson, S.E., Pearson, N.J., Griffi n, W.L., and Belousova, E.A., 2004, The application of laser ablation–induc-tively coupled plasma–mass spectrometry to in situ U-Pb zircon geochronology: Chemical Geology, v. 211, p. 47–69, doi:10.1016/j.chemgeo.2004.06.017.

Jaeckel, P., Kröner, A., Kamo, S.L., Brandl, G., and Wendt, J.I., 1997, Late Archaean to early Proterozoic granitoid magmatism and high-grade metamorphism in the cen-tral Limpopo belt, South Africa: Journal of the Geologi-cal Society of London, v. 154, p. 25–44, doi:10.1144/gsjgs.154.1.0025.

Jaffey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C., and Essling, A.M., 1971, Precision measurement of half-lives and specifi c activities of 235U and 238U: Physical Reviews, Section C: Nuclear Physics, v. 4, p. 1889–1906.

Jahn, B.-M., and Cuvellier, H., 1994, Pb-Pb and U-Pb geo-chronology of carbonate rocks: An assessment: Chemi-cal Geology, v. 115, p. 125–151, doi:10.1016/0009-2541(94)90149-X.

Jercinovic, M.J., and Williams, M.L., 2005, Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: Background ac-quisition interferences, and beam irradiation effects: The American Mineralogist, v. 90, p. 526–546, doi:10.2138/am.2005.1422.

Kamo, S.L., Gower, C.F., and Krogh, T.E., 1989, Birthdate for the Iapetus Ocean? A precise U-Pb zircon and baddeleyite age for the Long Range dikes, southeast Labrador: Geology, v. 17, p. 602–605, doi:10.1130/0091-7613(1989)017<0602:BFTLOA>2.3.CO;2.

Kamo, S.L., Czamanske, G.K., and Krogh, T.E., 1996a, A minimum U-Pb age for Siberian fl ood-basalt volcanism: Geochimica et Cosmochimica Acta, v. 60, p. 3505–3511, doi:10.1016/0016-7037(96)00173-1.

Kamo, S.L., Reimold, W.U., Krogh, T.E., and Colliston, W.P., 1996b, A 2.023 Ga age for the Vredefort impact event and a fi rst report of shock metamorphosed zircons in pseudotachylitic breccias and granophyre: Earth and Planetary Science Letters, v. 144, p. 369–387, doi:10.1016/S0012-821X(96)00180-X.

Kamo, S.L., Czamanske, G.K., Amelin, Y., Fedorenko, V.A., Davis, D.W., and Trofi mov, V.R., 2003, Rapid erup-tion of Siberian fl ood-volcanic rocks, coincident with the Permian-Triassic boundary and mass extinction at 251 Ma: Earth and Planetary Science Letters, v. 214, p. 75–91, doi:10.1016/S0012-821X(03)00347-9.

Kamo, S.L., Corfu, F., Heaman, L.M., and Moser, D.E., 2011, The Krogh revolution: Advances in the mea-surement of time: Canadian Journal of Earth Sciences, v. 48, p. 1–8, doi:10.1139/E11-003.

Kemp, A.I.S., Foster, G.L., Scherstén, A., Whitehouse, M.J., Darling, J., and Storey, C., 2009, Concurrent Pb-Hf iso-tope analysis of zircon by laser ablation multi-collector ICP-MS, with implications for the crustal evolution

of Greenland and the Himalayas: Chemical Geology, v. 261, p. 244–260, doi:10.1016/j.chemgeo.2008.06.019.

Kemp, A.I.S., Wilde, S.A., Hawkesworth, C.J., Coath, C.D., Nemchin, A., Pidgeon, R.T., Vervoort, J.D., and DuFrane , S.A., 2010, Hadean crustal evolution revisited: New constraints from Pb-Hf isotope systematics of the Jack Hills zircons: Earth and Planetary Science Letters, v. 296, p. 45–56, doi:10.1016/j.epsl.2010.04.043.

Kinny, P.D., and Dawson, J.B., 1992, A mantle metasomatic injection event linked to late Cretaceous kimberlite magmatism: Nature, v. 360, p. 726–728, doi:10.1038/360726a0.

Kober, B., 1986, Whole-grain evaporation for 207Pb/206Pb age investigations on single zircons using a double-fi lament thermal ion source: Contributions to Miner-alogy and Petrology, v. 93, p. 482–490, doi:10.1007/BF00371718.

Kober, B., 1987, Single zircon evaporation combined with Pb+ emitter bedding for 207Pb/206Pb age investigations using thermal ion mass spectrometry, and implications for zirconology: Contributions to Mineralogy and Petrol ogy, v. 96, p. 63–71, doi:10.1007/BF00375526.

Košler, J., Fonneland, H., Sylvester, P., Tubrett, M., and Pedersen, R.-B., 2002, U-Pb dating of detrital zircons for sediment provenance studies—A comparison of laser ablation ICP-MS and SIMS techniques: Chemi-cal Geology, v. 182, p. 605–618, doi:10.1016/S0009-2541(01)00341-2.

Kouvo, O., and Tilton, G.R., 1966, Mineral ages from the Finnish Precambrian: Journal of Geology, v. 74, p. 421–442.

Kramers, J., and Mouri, H., 2011, The Geochronology of the Limpopo Complex: A Controversy Solved, in van Reenen, D.D., Kramers, J.D., McCourt, S., and Per-chuk, L.L., eds., Origin and Evolution of Precambrian High-Grade Gneiss Terranes, with Special Emphasis on the Limpopo Complex of Southern Africa: Geo-logical Society of America Memoir 207, p. 85–106, doi:10.1130/2011.1207(06).

Kramers, J., Frei, R., Newville, M., Kober, B., and Villa, I., 2009, On the valency state of radiogenic lead in zir-con and its consequences: Chemical Geology, v. 261, p. 4–11, doi:10.1016/j.chemgeo.2008.09.010.

Krogh, T.E., 1973, A low contamination method for the hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations: Geo-chimica et Cosmochimica Acta, v. 37, p. 485–494, doi:10.1016/0016-7037(73)90213-5.

Krogh, T.E., 1982a, Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique: Geochimica et Cos-mochimica Acta, v. 46, p. 637–649, doi:10.1016/0016-7037(82)90165-X.

Krogh, T.E., 1982b, Improved accuracy of U-Pb zircon dating by selection of more concordant fractions using a high-gradient magnetic separation technique: Geochimica et Cosmochimica Acta, v. 46, p. 631–635, doi:10.1016/0016-7037(82)90164-8.

Krogh, T.E., 1994, Precise U-Pb ages for Grenvillian and pre-Grenvillian thrusting of Proterozoic and Archean metamorphic assemblages in the Grenville Front tec-tonic zone, Canada: Tectonics, v. 13, p. 963–982, doi:10.1029/94TC00801.

Krogh, T.E., and Davis, G.L., 1974, Alteration in zircons with discordant U-Pb ages: Carnegie Institution Wash-ington Yearbook, v. 73, p. 560–567.

Krogh, T.E., and Davis, G.L., 1975a, The production and preparation of 205Pb for use as a tracer for isotope di-lution analyses: Carnegie Institution Washington Year-book, v. 74, p. 416–417.

Krogh, T.E., and Davis, G.L., 1975b, Alteration in zircons and differential dissolution of altered and metamict zir-con: Carnegie Institution Washington Yearbook, v. 74, p. 619–623.

Krogh, T.E., Corfu, F., Davis, D.W., Dunning, D.R., Hea-man, L.M., Kamo, S.L., Machado, N., Greenough, J.D., and Nakamura, E., 1987, Precise U-Pb iso-topic ages of diabase dykes and mafi c to ultramafi c rocks using trace amounts of baddeleyite and zircon, in Halls, H.C., and Fahrig, W.F., eds., Mafi c Dyke Swarms: Geological Association of Canada Special Paper 34, p. 147–152.



Corfu


Krogh, T.E., Kamo, S.L., Sharpton, V., Marin, L., and Hilde-brand, A.R., 1993, U-Pb ages of single shocked zircons linking distal K/T ejecta to the Chicxulub crater: Na-ture, v. 366, p. 731–734, doi:10.1038/366731a0.

Kröner, A., Byerly, G.R., and Lowe, D.R., 1991, Chronol-ogy of Early Archean granite-greenstone evolution in the Barberton Mountain Land, South Africa, based on precise dating by single zircon evaporation: Earth and Planetary Science Letters, v. 103, p. 41–54, doi:10.1016/0012-821X(91)90148-B.

Kulp, J.L., Bate, G.L., and Broecker, W.S., 1954, Pres-ent status of the lead method of age determination: American Journal of Science, v. 252, p. 345–365, doi:10.2475/ajs.252.6.345.

Lancelot, J.R., Vitrac, A., and Allègre, C.J., 1976, Uranium and lead isotopic dating with grain-by-grain analysis: A study of complex geological history with a single rock: Earth and Planetary Science Letters, v. 29, p. 357–366, doi:10.1016/0012-821X(76)90140-0.

Lima, S.M., Corfu, F., Neiva, A.M.R., and Ramos, J.M.F., 2012, Dissecting complex magmatic processes: An in-depth U-Pb study of the Pavia Pluton, Ossa Morena zone, Portugal: Journal of Petrology, v. 53, p. 1887–1911, doi:10.1093/petrology/egs037.

Ludwig, K.R., 1998, On the treatment of concordant uranium-lead ages: Geochimica et Cosmochimica Acta, v. 62, p. 665–676, doi:10.1016/S0016-7037(98)00059-3.

Mattinson, J.M., 1972, Preparation of hydrofl uoric, hydro-chloric, and nitric acids at ultra-low lead levels: Ana-lytical Chemistry, v. 44, p. 1715–1716, doi:10.1021/ac60317a032.

Mattinson, J.M., 1994, A study of complex discordance in zircons using step-wise dissolution techniques: Contri-butions to Mineralogy and Petrology, v. 116, p. 117–129, doi:10.1007/BF00310694.

Mattinson, J.M., 2005, Zircon U-Pb chemical abrasion (“CA–TIMS”) method: Combined annealing and multi-step dissolution analysis for improved precision and accuracy of zircon ages: Chemical Geology, v. 220, p. 47–66, doi:10.1016/j.chemgeo.2005.03.011.

Mattinson, J.M., 2010, Analysis of the relative decay con-stants of 235U and 238U by multi-step CA-TIMS mea-surements of closed-system natural zircon samples: Chemical Geology, v. 275, p. 186–198, doi:10.1016/j.chemgeo.2010.05.007.

Mattinson, J.M., 2011, Extending the Krogh legacy: Devel-opment of the CA-TIMS method for zircon geochro-nology: Canadian Journal of Earth Sciences, v. 48, p. 95–105.

Mattinson, J.M., Graubard, C.M., Parkinson, D.L., and McLelland, W.C., 1996, U-Pb reverse discordance in zircons: The role of fi ne-scale oscillatory zoning and sub-microscopic transport of Pb: American Geophysi-cal Union Geophysical Monograph 95, p. 355–370, doi:10.1029/GM095p0355.

Matzel, J., Bowring, S., and Miller, R., 2006, Time scales of pluton construction at differing crustal levels: Examples from the Mount Stuart and Tenpeak intrusions, North Cascades, Washington: Geological Society of America Bulletin, v. 118, p. 1412–1430, doi:10.1130/B25923.1.

McFarlane, C.R.M., Connelly, J.N., and Carlson, W.D., 2005, Intracrystalline redistribution of Pb in zircon during high-temperature metamorphism: Chemical Geology, v. 217, p. 1–28, doi:10.1016/j.chemgeo.2004.11.019.

McLaren, A.C., FitzGerald, J.D., and Williams, I.S., 1994, The microstructure of zircon and its infl uence on the age determination from U-Pb isotopic ratios measured by ion microprobe: Geochimica et Cosmochimica Acta, v. 58, p. 993–1005, doi:10.1016/0016-7037(94)90521-5.

Medenbach, O., 1976, Geochemie der Elemente in Zirkon und ihre räumliche Verteilung—Eine Untersuchung mit der Elektronenstrahlmikrosonde [Doctoral thesis]: Heidel-berg, Germany, Ruprecht-Karl-University. 58 pages.

Mezger, K., and Krogstad, E.J., 1997, Interpretation of dis-cordant U-Pb zircon ages: An evaluation: Journal of Metamorphic Geology, v. 15, p. 127–140, doi:10.1111/j.1525-1314.1997.00008.x.

Miller, J.S., Matzel, J.P., Miller, C.F., Burgess, S.D., and Miller, R.B., 2007, Zircon growth and recycling dur-ing the assembly of large, composite arc plutons: Jour-nal of Volcanology and Geothermal Research, v. 167, p. 282–299, doi:10.1016/j.jvolgeores.2007.04.019.

Montel, J., Foret, S., Veschambre, M., Nicollet, C., and Provost, A., 1996, Electron microprobe dating of monazite: Chemical Geology, v. 131, p. 37–53, doi:10.1016/0009-2541(96)00024-1.

Moorbath, S., Taylor, P.N., Orpen, J.L., Treloar, P., and Wilson, J.F., 1987, First direct radiometric dating of Archean stromatolitic limestone: Nature, v. 326, p. 865–867, doi:10.1038/326865a0.

Moser, D.E., and Heaman, L.M., 1997, Proterozoic zircon growth in Archean lower crustal xenoliths, southern Superior craton—A consequence of Matachewan ocean opening: Contributions to Mineralogy and Petrology, v. 128, p. 164–175, doi:10.1007/s004100050301.

Moser, D.E., Davis, W.J., Reddy, S.M., Flemming, R.L., and Hart, R.J., 2009, Zircon U-Pb strain chronometry re-veals deep impact triggered fl ow: Earth and Planetary Science Letters, v. 277, p. 73–79, doi:10.1016/j.epsl.2008.09.036.

Moser, D.E., Cupelli, C.L., Barker, I.R., Flowers, R.M., Bowman, J.R., Wooden, J., and Hart, J.R., 2011, New zircon shock phenomena and their use for dating and reconstruction of large impact structures revealed by electron nanobeam (EBSD, CL, EDS) and isotopic U-Pb and (U-Th)/He analysis of the Vredefort dome: Canadian Journal of Earth Sciences, v. 48, p. 117–139, doi:10.1139/E11-011.

Mundil, R., Metcalfe, I., Ludwig, K.R., Renne, P.R., Oberli, F., and Nicoll, R.S., 2001, Timing of the Permian-Triassic biotic crisis: Implications from new zircon U/Pb age data (and their limitations): Earth and Plan-etary Science Letters, v. 187, p. 131–145, doi:10.1016/S0012-821X(01)00274-6.

Nasdala, L., Hanchar, J.M., Rhede, D., Kennedy, A.K., and Váczi, T., 2010, Retention of uranium in complexly altered zircon: An example from Bancroft, Ontario: Chemical Geology, v. 269, p. 290–300, doi:10.1016/j.chemgeo.2009.10.004.

Nemchin, A.A., Pidgeon, R.T., Whitehouse, M., Vaughan, J.P., and Meyer, C., 2008, SIMS U-Pb study of zircon from Apollo 14 and 17 breccias: Implications for the evolution of lunar KREEP: Geochimica et Cosmo-chimica Acta, v. 72, p. 668–689, doi:10.1016/j.gca.2007.11.009.

Nemchin, A., Timms, N., Pidgeon, R., Geisler, T., Reddy, S., and Meyer, C., 2009, Timing of crystallization of the lunar magma ocean constrained by the oldest zir-con: Nature Geoscience, v. 2, p. 133–136, doi:10.1038/ngeo417.

Neymark, L.A., Amelin, Y.V., and Paces, J.B., 2000, 206Pb-230Th-234U-238U and 207Pb-235U geochronology of Quaternary opal, Yucca Mountain, Nevada: Geo chimica et Cosmochimica Acta, v. 64, p. 2913–2928, doi:10.1016/S0016-7037(00)00408-7.

Nicolaysen, L.O., 1957, Solid diffusion in radioactive miner-als and the measurement of absolute age: Geo chimica et Cosmochimica Acta, v. 11, p. 41–59, doi:10.1016/0016-7037(57)90004-2.

Nier, A.O., 1939, The isotopic composition of radio-genic leads and the measurement of geological time: Physical Review, v. 55, p. 153–163, doi:10.1103/PhysRev.55.153.

Nutman, A.P., McGregor, V.R., Friend, C.R.L., Bennett, V.C., and Kinny, P.D., 1996, The Itsaq Gneiss Complex of southern West Greenland: The world’s most exten-sive record of early crustal evolution (3900–3600 Ma): Precambrian Research, v. 78, p. 1–39, doi:10.1016/0301-9268(95)00066-6.

Parrish, R.R., 1990, U-Pb dating of monazite and its applica-tion to geological problems: Canadian Journal of Earth Sciences, v. 27, p. 1431–1450, doi:10.1139/e90-152.

Parrish, R.R., 1995, Thermal and tectonic evolution of the southeastern Canadian Cordillera: Canadian Journal of Earth Sciences, v. 32, p. 1618–1642, doi:10.1139/e95-130.

Parrish, R.R., and Noble, S.R., 2003, Zircon U-Pb geo-chronology by isotope dilution–thermal ionisation mass spectrometry (ID-TIMS), in Hanchar, J.M., and Hoskins, P.W.O., eds., Zircon: Reviews in Mineralogy and Geochemistry, v. 53, p. 183–213.

Patterson, C.C., 1956, Age of meteorites and the Earth: Geochimica et Cosmochimica Acta, v. 10, p. 230–237, doi:10.1016/0016-7037(56)90036-9.

Pidgeon, R.T., and Aftalion, M., 1978, Cogenetic and in-herited zircon U-Pb systems in granites: Palaeozoic granites of Scotland and England, in Bowes, D.R., and Leake, B.E., eds., Crustal Evolution in Northwestern Britain and Adjacent Regions: Geological Journal, Special Issue 10, p. 183–248.

Pidgeon, R.T., and Hopgood, A.M., 1975, Geochronol-ogy of Archaean gneisses and tonalites from north of the Frederiksåbs isblink, S.W. Greenland: Geo-chimica et Cosmochimica Acta, v. 39, p. 1333–1346, doi:10.1016/0016-7037(75)90113-1.

Pidgeon, R.T., O’Neill, J., and Silver, L.T., 1966, Uranium and lead isotopic stability in a metamict zircon under experimental hydrothermal conditions: Science, v. 154, p. 1538–1540, doi:10.1126/science.154.3756.1538.

Pidgeon, R.T., Nemchin, A.A., and Hitchen, G.J., 1998, In-ternal structures of zircons from Archean granites from the Darling Range batholith: Implications for zircon stability and the interpretation of zircon U-Pb ages: Contributions to Mineralogy and Petrology, v. 132, p. 288–299, doi:10.1007/s004100050422.

Poitrasson, F., Chenery, S., and Bland, D.J., 1996, Con-trasted monazite hydrothermal alteration mechanisms and their geochemical implications: Earth and Plan-etary Science Letters, v. 145, p. 79–96, doi:10.1016/S0012-821X(96)00193-8.

Ramezani, J., Schmitz, M.D., Davydov, V.I., Bowring, S.A., Snyder, W.S., and Northrup, C.J., 2007, High-precision U-Pb zircon age constraints on the Carboniferous-Permian boundary in the Southern Urals stratotype: Earth and Planetary Science Letters, v. 256, p. 244–257, doi:10.1016/j.epsl.2007.01.032.

Reddy, S.M., Timms, N.E., Trimby, P.W., Kinny, P.D., Buchan , C., and Blake, K., 2006, Crystal-plastic defor-ma tion of zircon: A defect in the assumption of chemi-cal robustness: Geology, v. 34, p. 257–260, doi:10.1130/G22110.1.

Reid, M.R., Coath, C.D., Harrison, T.M., and McKeegan, K.D., 1997, Prolonged residence times for the young-est rhyolites associated with Long Valley caldera; 230Th-238U ion microprobe dating of young zircons: Earth and Planetary Science Letters, v. 150, p. 27–39, doi:10.1016/S0012-821X(97)00077-0.

Roffeis, C., Corfu, F., and Austrheim, H., 2012, Evidence for a Caledonian amphibolite to eclogite facies pres-sure gradient in the Middle Allochthon Lindås Nappe, SW Norway: Contributions to Mineralogy and Petrol-ogy, v. 164, p. 81–99.

Romer, R.L., 2003, Alpha-recoil in U-Pb geochronology: Effective sample size matters: Contributions to Min-eralogy and Petrology, v. 145, p. 481–491, doi:10.1007/s00410-003-0463-0.

Rubatto, D., 2002, Zircon trace element geochemistry: Parti-tioning with garnet and the link between U-Pb ages and metamorphism: Chemical Geology, v. 184, p. 123–138, doi:10.1016/S0009-2541(01)00355-2.

Russell, R.D., and Ahrens, L.H., 1957, Additional regulari-ties among discordant lead-uranium ages: Geochimica et Cosmochimica Acta, v. 11, p. 213–218, doi:10.1016/0016-7037(57)90095-9.

Rybach, L., 1973, Wärmeproduktionsbestimmungen an Ges-teinen der Schweizer Alpen: Beitrag zur Geologie der Schweiz, Geotechnische Serie 51, 43 p.

Rybach, L., and Labhart, T.P., 1973, Regelmässigkeiten der Radioaktivitätsverteilung in granitischen Ges-teinskörpern (Beispiele aus den Schweizer Alpen): Schweizerische Mineralogische und Petrographische Mitteilungen, v. 53, p. 379–384.

Schaltegger, U., Bartolini, A., Guex, J., Schoene, B., and Ovtcharova, M., 2008, Precise U-Pb age constraints for end-Triassic mass extinction, its correlation to vol-canism and Hettangian post-extinction recovery: Earth and Planetary Science Letters, v. 267, p. 266–275, doi:10.1016/j.epsl.2007.11.031.

Schärer, U., 1980, U-Pb and Rb-Sr dating of a polymetamor-phic nappe terrain: The Caledonian Jotun nappe, south-ern Norway: Earth and Planetary Science Letters, v. 49, p. 205–218, doi:10.1016/0012-821X(80)90065-5.

Schärer, U., 1984, The effect of initial 230Th disequilibrium on young U-Pb ages: The Makalu case, Himalaya: Earth and Planetary Science Letters, v. 67, p. 191–204, doi:10.1016/0012-821X(84)90114-6.





Schärer, U., and Allègre, C.J., 1982, Investigation of the Archean crust by single-grain dating of detrital zircon: A greywacke of the Slave Province, Canada: Cana-dian Journal of Earth Sciences, v. 19, p. 1910–1918, doi:10.1139/e82-169.

Schmitt, A.K., 2006, Laacher See revisited: High spatial resolution zircon dating indicates rapid formation of a zoned magma chamber: Geology, v. 34, p. 597–600, doi:10.1130/G22533.1.

Schmitt, A.K., 2007, Ion microprobe analysis of (231Pa)/(235U) and an appraisal of protactinium partitioning in igne-ous zircon: The American Mineralogist, v. 92, p. 691–694, doi:10.2138/am.2007.2449.

Schmitz, M.D., and Bowring, S.A., 2001, U-Pb zircon and titanite systematics of the Fish Canyon Tuff: An as-sessment of high precision U-Pb geochronology and its application to young volcanic rocks: Geochimica et Cosmochimica Acta, v. 65, p. 2571–2587, doi:10.1016/S0016-7037(01)00616-0.

Schoene, B., Crowley, J.L., Condon, D.C., Schmitz, M.D., and Bowring, S.A., 2006, Reassessing the uranium decay constants for geochronology using ID-TIMS U-Pb data: Geochimica et Cosmochimica Acta, v. 70, p. 426–445, doi:10.1016/j.gca.2005.09.007.

Schoene, B., Guex, J., Bartolini, A., Schaltegger, U., and Blackburn, T.J., 2010a, Correlating the end-Triassic mass extinction and fl ood basalt volcanism at the 100,000-year level: Geology, v. 38, p. 387–390, doi:10.1130/G30683.1.

Schoene, B., Latkoczy, C., Schaltegger, U., and Günther, D., 2010b, A new method integrating high-precision U-Pb geochronology with zircon trace element analysis (U-Pb TIMS-TEA: Geochimica et Cosmochimica Acta, v. 74, p. 7144–7159, doi:10.1016/j.gca.2010.09.016.

Shen, S., Crowley, J.L., Wang, Y., et al., 2011, Calibrating the end-Permian mass extinction: Science, v. 334, p. 1367–1372, doi:10.1126/science.1213454.

Silver, L.T., 1963a, The relation between radioactivity and discordance in zircons: Nuclear Geophysics, National Research Council Publication 1075, p. 34–39.

National Academy of Sciences, Natural Research Council, Nuclear Geophysics Publication 1075, p. 34–39.

Silver, L.T., 1963b, The use of cogenetic uranium-lead iso-tope systems in zircons in geochronology, in Radio-active Dating: International Atomic Energy Agency Proceedings: Vienna, Austria, Internal Atomic Energy Agency, p. 279–287.

Silver, L.T., and Deutsch, S., 1963, Uranium-lead isotopic variations in zircon; a case study: The Journal of Geol-ogy, v. 71, p. 721–758, doi:10.1086/626951.

Steiger, R.H., and Jäger, E., 1977, Subcommission on Geo-chronology—Convention on use of decay constants in geochronology and cosmochronology: Earth and Plan-etary Science Letters, v. 36, p. 359–362, doi:10.1016/0012-821X(77)90060-7.

Steiger, R.H., and Wasserburg, G.J., 1966, Systematics in the208Pb-232Th, 207Pb-235U, and 206Pb-238U systems: Jour-nal of Geophysical Research, v. 71, p. 6065–6090, doi:10.1029/JZ071i024p06065.

Steiger, R.H., and Wasserburg, G.J., 1969, Comparative U-Th-Pb systematics in 2.7 × 109 yr plutons of differ-ent geologic histories: Geochimica et Cosmochimica Acta, v. 33, p. 1213–1232, doi:10.1016/0016-7037(69)90043-X.

Steiger, R.H., Bickel, R.A., and Meier, M., 1993, Conven-tional U-Pb dating of single fragments of zircon for petrogenetic studies of Phanerozoic granitoids: Earth and Planetary Science Letters, v. 115, p. 197–209, doi:10.1016/0012-821X(93)90222-U.

Stern, R., Bodorkos, S., Kamo, S., Hickman, A., and Corfu, F., 2009, Measurement of SIMS instrumental mass

fractionation of Pb-isotopes during zircon dating: Geo-standards and Geoanalytical Research, v. 33, p. 145–168, doi:10.1111/j.1751-908X.2009.00023.x.

Suzuki, K., and Adachi, M., 1991, Precambrian provenance and Silurian metamorphism of the Tsubonosawa para-gneiss in the South Kitakami terrane, northeast Japan, revealed by the chemical Th–U–total Pb isochron ages of monazite, zircon and xenotime: Geochemical Jour-nal, v. 25, p. 357–376, doi:10.2343/geochemj.25.357.

Svensen, H., Corfu, F., Polteau, S., Hammer, Ø., and Planke, S., 2012, Rapid magma emplacement in the Karoo Large Igneous Province: Earth and Planetary Science Letters, v. 325–326, p. 1–9, doi:10.1016/j.epsl.2012.01.015.

Tatsumoto, M., Knight, R.J., and Allègre, C.J., 1973, Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206: Science, v. 180, p. 1279–1283, doi:10.1126/science.180.4092.1279.

Tera, F., and Wasserburg, G.J., 1972, U-Th-Pb systematics in three Apollo 14 basalts and the problem of initial Pb in lunar rocks: Earth and Planetary Science Letters, v. 14, p. 281–304, doi:10.1016/0012-821X(72)90128-8.

Tera, F., and Wasserburg, G.J., 1975, Precise isotopic analysis of lead in picomole and subpicomole quanti-ties: Ana lytical Chemistry, v. 47, p. 2214–2220, doi:10.1021/ac60363a036.

Tilton, G.R., 1960, Volume diffusion as a mechanism for discordant lead ages: Journal of Geophysical Research, v. 65, p. 2933–2963, doi:10.1029/JZ065i009p02933.

Tilton, G.R., 1973, Isotopic lead ages of chondritic meteor-ites: Earth and Planetary Science Letters, v. 19, p. 321–329, doi:10.1016/0012-821X(73)90082-4.

Tilton, G.R., and Grünenfelder, M., 1968, Sphene; uranium lead ages: Science, v. 159, p. 1458–1461, doi:10.1126/science.159.3822.1458.

Tilton, G.R., Patterson, C., Brown, H., Inghram, M., Hayden, R., Hess, D., and Larsen, E., 1955, Isotopic composi-tion and distribution of lead, uranium and thorium in a Precambrian granite: Geological Society of America Bulletin, v. 66, p. 1131–1148, doi:10.1130/0016-7606(1955)66[1131:ICADOL]2.0.CO;2.

Todt, W.A., and Büsch, W., 1981, U-Pb investigations on zircons from pre-Variscan gneisses: I. A study of the Schwarzwald, West Germany: Geo chimica et Cosmo-chimica Acta, v. 45, p. 1789–1801, doi:10.1016/0016-7037(81)90010-7.

Tucker, R.D., Råheim, A., Krogh, T.E., and Corfu, F., 1987, Uranium-lead zircon and titanite ages from the north-ern portion of the Western Gneiss Region, south-central Norway: Earth and Planetary Science Letters, v. 81, p. 203–211, doi:10.1016/0012-821X(87)90156-7.

Tucker, R.D., Krogh, T.E., and Råheim, A., 1990, Protero-zoic evolution and age-province boundaries in the central part of the Western Gneiss Region, Norway: Results of U-Pb dating of accessory minerals from Trondheimsfjord to Geiranger, in Gower, C.F., Rivers, T., and Ryan, B., eds., Mid-Proterozoic Geology of the Southern Margin of Proto-Laurentia–Baltica: Geologi-cal Association of Canada Special Paper 241, p. 33–50.

Turek, A., Smith, P.E., and Van Schmus, W.R., 1982, Rb-Sr and U-Pb ages of volcanism and granite emplacement in the Michipicoten belt, Wawa, Ontario: Canadian Jour-nal of Earth Sciences, v. 19, p. 1608–1626, doi:10.1139/e82-138.

Valley, J.W., Lackey, J.S., Cavosie, A.J., Clechenko, C.C., Spicuzza, M.J., Basei, M.A.S., Bindeman, I.N., Fer-reira, V.P., Sial, A.N., King, E.M., Peck, W.H., Sinha, A.K., and Wei, C.S., 2005, 4.4 billion years of crustal maturation: Oxygen isotopes in magmatic zircon: Con-tributions to Mineralogy and Petrology, v. 150, p. 561–580, doi:10.1007/s00410-005-0025-8.

Van Schmus, W.R., Bickford, M.E., Lewry, J.F., and Mac-donald, R., 1987, U-Pb geochronology of the Trans-Hudson orogen in northern Saskatchewan, Canada: Canadian Journal of Earth Sciences, v. 24, p. 407–424, doi:10.1139/e87-043.

Wasserburg, G.J., 1963, Diffusion processes in lead-uranium systems: Journal of Geophysical Research, v. 68, p. 4823–4846.

Watson, E.B., and Harrison, T.M., 2005, Zircon thermom-eter reveals minimum melting conditions on earli-est Earth: Science, v. 308, p. 841–844, doi:10.1126/science.1110873.

Wetherill, G.W., 1956a, An interpretation of the Rhodesia and Witwatersrand age patterns: Geochimica et Cosmo-chimica Acta, v. 9, p. 290–292, doi:10.1016/0016-7037(56)90029-1.

Wetherill, G.W., 1956b, Discordant uranium-lead ages: Transactions of the American Geophysical Union, v. 37, p. 320–326.

Wetherill, G.W., 1963, Discordant uranium-lead ages—Part 2: Discordant ages resulting from diffusion of lead and uranium: Journal of Geophysical Research, v. 68, p. 2957–2965, doi:10.1029/JZ068i010p02957.

Whitehouse, M.J., and Kamber, B.S., 2005, Assigning dates to thin gneissic veins in high-grade metamorphic ter-ranes: A cautionary tale from Akilia, southwest Green-land: Journal of Petrology, v. 46, p. 291–318, doi:10.1093/petrology/egh075.

Whitehouse, M.J., and Nemchin, A.A., 2008, High preci-sion, high accuracy measurement of oxygen isotopes in a large lunar zircon by SIMS: Chemical Geology, v. 261, p. 32–42, doi:10.1016/j.chemgeo.2008.09.009.

Whitehouse, M.J., and Platt, J.P., 2003, Dating high-grade metamorphism—Constraints from rare-earth ele-ments in zircon and garnet: Contributions to Miner-alogy and Petrology, v. 145, p. 61–74, doi:10.1007/s00410-002-0432-z.

Wiedenbeck, M., 1995, An example of reverse discordance during ion microprobe zircon dating: An artifact of en-hanced ion yields from a radiogenic labile Pb: Chemi-cal Geology, v. 125, p. 197–218, doi:10.1016/0009-2541(95)00072-T.

Wilde, S.A., Valley, J.W., Peck, W.H., and Graham, C.M., 2001, Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago: Nature, v. 409, p. 175–178, doi:10.1038/35051550.

Williams, I.S., 1998, U-Th-Pb geochronology by ion micro-probe, in McKibben, M.A., Shanks W.C., III, and Ridley, W.I., eds., Applications of Microanalytical Techniques to Understanding Mineralizing Processes: Reviews in Economic Geology, v. 7, p. 1–35.

Williams, I.S., 2001, Response of detrital zircon and mona-zite, and their U-Pb isotopic systems, to regional metamorphism and host rock partial melting, Cooma Complex, southeastern Australia: Australian Journal of Earth Sciences, v. 48, p. 557–580, doi:10.1046/j.1440-0952.2001.00883.x.

Williams, I.S., Compston, W., Black, L.P., Ireland, T.R., and Foster, J.J., 1984, Unsupported radiogenic Pb in zircon: A case of anomalously high Pb-Pb, U-Pb and Th-Pb ages: Contributions to Mineralogy and Petrol-ogy, v. 88, p. 322–327, doi:10.1007/BF00376756.

SCIENCE EDITOR: BRENDAN MURPHY

MANUSCRIPT RECEIVED 21 MARCH 2012REVISED MANUSCRIPT RECEIVED 7 JULY 2012MANUSCRIPT ACCEPTED 11 JULY 2012

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A Century of U-Pb Geochronology

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